2021年4月3日雅思考試機(jī)經(jīng)真題答案回憶(3)

2023-06-07 13:01:12 來源：中國教育在線

2021年4月3日雅思考試機(jī)經(jīng)真題答案回憶(3) 關(guān)于這個問題下面小編就來為各個考生解答下。

4月3日的雅思考試回憶來啦！

READING

Passage 1

Topic

Footprints in the muds

A.EVERYBODY knows that the dinosaurs were killed by an asteroid. Something big hitthe earth 65 million years ago and, when the dust had fallen, so had the great reptiles.There is thus a nice if ironic，symmetry in the idea that a similar impact brought aboutthe dinosaurs' rise. That is the thesis proposed by Paul Olsen, of Columbia University,and his colleagues in this week's Science.

B. Dinosaurs first appeared in the fossil record 230m years ago，during the Triassicperiod.But they were mostly small, and they shared the earth with lots of other sorts ofreptiles. lt was in the subsequent Jurassic, which began 202 million years ago, that theyoverran the planet and turmed into the monsters depicted in the book and movie"Jurassic Park"(2).(Actually, though, the dinosaurs that appeared on screen were fromthe still more recent Cretaceous period.) Dr. Olsen and his colleagues are not the firstto suggest that the dinosaurs inherited the earth as the result of an asteroid strike.Butthey are the first to show that the takeover did, indeed, happen in a geological eyeblink.C. Dinosaur skeletons are rare. Dinosaur footprints are，however，surprisinglyabundant. ( 4)And the sizes of the prints are as good an indication of the sizes of thebeasts as are the skeletons themselves. Dr. Olsen and his colleagues,therefore,concentrated on prints, not bones.

D.The prints in question were made in eastern North America, a part of the world full ofrift valleys to those in East Africa today. Like the moderm African rift valleys,theTriassiclJurassic American ones contained lakes, and these lakes grew and shrank atregular intervals because of climatic changes caused by periodic shifts in the earth'sorbit. (A similar phenomenon is responsible for modem ice ages.)That regularity,combined with reversals in the earth's magnetic field, which are detectable in the tinyfields of certain magnetic minerals，means that rocks from this place and period can bedated to within a few thousand years.As a bonus,squishy lake edge sediments are justthe things for recording the tracks of passing animals. By dividing the labor betweenthemselves, the ten authors of the paper were able to study such tracks at 80 sites.

E. The researchers looked at 18 so-called ichnotaxa. These are recognizable types offootprint that cannot be matched precisely with the species of animals that left them.(6)But they can be matched with a general sort of animal, and thus act as an indicator ofthe fate of that group，even when there are no bones to tell the story. Five of theichnotaxa disappear before the end of the Triassic, and four march confidently acrossthe boundary into the Jurassic.(5)Six,however, vanish at the boundary , or only justsplutter across it; and there appear from nowhere，almost as soon as the Jurassicbegins.

F.That boundary itself is suggestive. The first geological indication of the impact thatkilled the dinosaurs was an unusually high level of iridium in rocks at the end of theCretaceous when the beasts disappear from the fossil record. lridium is normally rare atthe earth's surface,but it is more abundant in meteorites. When people began tobelieve the impact theory,they started looking for other Cretaceous-and anomalies.One that turned up was a surprising abundance of ferm spores in rocks just above theboundary layer - a phenomenon known as a "fern spike".

G. That matched the theory nicely. Many modem ferns are opportunists. They cannotcompete against plants with leaves, but if a piece of land is cleared by, say, a volcaniceruption,they are often the first things to set up shop there.An asteroid strike wouldhave scoured much of the earth of its vegetable cover,and provided a paradise forferns. A fem spike in the rocks is thus a good indication that something terrible hashappened.

H.Both an iridium anomaly and a ferm spike appear in rocks at the end of the Triassic,too.That accounts for the disappearing ichnotaxa: the creatures that made them did notsurvive the holocaust.The surprise is how rapidly the new ichnotaxa appears.

l. Dr. Olsen and his colleagues suggest that the explanation for this rapid increase insize may be a phenomenon called ecological release. This is seen today when reptiles(which, in modem times, tend to be small creatures) reach islands where they face nocompetitors. The most spectacular example is on the Indonesian island of Komodo,where local lizards have grown so large that they are often referred to as dragons.Thedinosaurs, in other words, could flourish only when the competition had been knocked

out.

J.That leaves the question of where the impact happened.No large hole in the earth'scrust seems to be 202m years old. lt may, of course, have been overlooked. Old cratersare eroded and buried, and not always easy to find. Alternatively, it may have vanished.Although the continental crust is more or less permanent, the ocean floor is constantlyrecycled by the tectonic processes that bring about continental drift There is no oceanfloor left that is more than 200m years old, so a crater that formed in the ocean wouldhave been swallowed up by now.

K. There is a third possibility, however. This is that the crater is known, but has beenmisdated. The Manicouagan"structure", a crater in Quebec, is thought to be 214myears old. lt is huge - some 100km across - and seems to be the largest of betweenthree and five craters that formed within a few hours of each other as the lumps of adisintegrated comet hit the earth one by one.

1-6為判斷題

1.There's still doubts for the theory that the asteroid strike led to the disappearance ofdinosaurs.(Y )

2.The size of the dinosaurs are often exaggerated by the books and movie.(N)3.Other scientists rejected the theory of Dr Oslen.(NG)

4.Dinosaur footprints are more found more adequately than dinosaur skeletons.(Y)5.Several species of dinosaur have lived from Triassic to Jurassic.(Y)

6 lchnotaxa offers the exact identification of the species of dinosaurs.(N)7-13為填空題

Dr. Olsen and his colleagues applied a phenomenon named 7 ...ecological release...toexplain the large size of the Eubrontes, which is a similar case to that nowadays reptilesinvade a place where there are no 8...competitors..; for example, on an island calledKomodo，indigenous huge lizards grow so big that people even regarding them as9...dragons... However, there were no old impact trace being found? The answer maybe that we have 10...overlooked... the evidence. Old craters are difficult to spot or itprobably have11...vanished... due to the effect of the earth moving. The ocean floor is regularly recycled by the 12...tectonic processes...that produce the continental drift.Besides,the third hypothesis is that the potential evidence - some craters maybe13...misdated ...

Passage 2

Topic

The Evolution of the hum ble heirloom tomatoes

AFamous for their taste, color and,well，homeliness，heirloom tomatoes tug at theheartstrings of gardeners and advocates of locally grown foods. The tomato aficionadomight conclude that, given the immense varieties—which go by such fanciful names asAunt Gertie's Gold and the Green Zebra—heirlooms must have a more diverse andsuperior set of genes than their grocery store cousins, those run-of-the-mill hybrid varietiessuch as beefsteak, cherry and plum. No matter how you slice it, however,their seemingdiversity is only skin-deep: heirlooms are actually feeble and inbred."the irony of all this,"says Steven Tanksley, a geneticist at Cornell University, "is all that diversity of heirloomscan be accounted for by a handful of genes.There's probably no more than 10 mutantgenes that create the diversity of heirlooms you see." But rather than simply debunking amyth about the heirloom's diversity，Tanksley's deconstruction of the tomato genome,along with work by others, is showing how an unassuming berry from the Andes becameone of the world's top crops.Genetics work will also point the way to sturdier , more flavorfultomatoes—albeit hybrid varieties whose seedscannot be passed down from generation togeneration but must be purchased anew by growers each season.

BThe cultivated tomato is a member of the nightshade family that includes New Worldcrops such as potatoand chili pepper，which spread around the world after ChristopherColumbus brought them back to Spain in the 15th century.* But whereas scientists haveuncovered a wealth of archaeological evidence—including microscopic starches on potteryshards that point to the taming of many crops from the Americas as far back as 10,00oyears ago—the record is blank when it comes to the tomato.Known scientificallyas Solanum lycopersicum，the modem tomato seems to have its wild origins in the Peruvian Andes and may have been domesticated in Vera Cruz,Mexico-an agriculturalhot spot Primitive varieties still grow throughout the Americas. All told , botanists call some13 species "tomatoes" and consider an additional four to be close allies.One mightassume that one of these known wild species became todays cultivated crop, but that's notthe case: the Mother Tomato has never been found. The closest relative is the curranttomato—Solanum pimpinellifolium—which，based on genetic comparisons，split fromtoday's tomato some 1.4 million years ago.

So researchers like Tanksley have to work backward,crossing tomato varieties andspecies in order to understand how various genes influence shape and size.Onceisolated，Tanksley later inserts those genes into other tomato varieties to make his casewith a dramatic transformation.Tanksley concludes from his analyses that, in their effort tomake bigger，tastier and faster-growing fruit,our ancestors ultimately exploited just 3omutations out of the tomato's 35,000 genes.Most of these genes have only small effectson tomato size and shape, but last May in Nature Genetics Tanksley and his colleaguesreported that they found a gene they dubbed fasciated that bumps up fruit size by 5opercent.It was probably the single most important event in domestication.The first writtenrecord of tomatoes—from Spain in the 1500s—-confirms that this mutation,which enlargestomatoes by producing compartments known as locules, existed back in the same yellowtomatoes that gave ltalians the word pomodoro, or golden apple. A cherry tomato typicallyhas two compartments filled with seeds and jelly,whereas a Jumbo Red can have up toeight locules. This gene，along with another size-goverming gene called fw2.2,whichTanksley identified 10 years earlier, was the key step in making tomatoes a dietary staple.

Besides size, tomato farmers also selected for shape.To discover those genes, Esthervan der Knaap, a Tanksley alumnus now at The Ohio State University，says she wentstraight for the heirlooms, which exhibit a range from the Jersey Devil's small, chili-peppershape to plump,cracked beefsteaks known as Radiator Charlie's Mortgage Lifter. "l justwent to catalogues and ordered everything that had a cute shape," she says.She pluckeda gene called SUN from one heirloom tomato and inserted it into a wild relative.As aresult, the tiny fruits bulged like pears, a remarkable makeover that made the cover of the joumal Sciencelast March. SUN'seffect dwarfsthat of another shape genecalled OVATE—yet another Tanksley discovery—and both seem to have been nurtured inEurope in the last several hundred years to ease mechanical harvesting and processing.

The selection of these traits has taken a toll on the heirloom's hardiness: They are oftenplagued by fungal infections that cause the fruit to crack,split and otherwise rot quickly.Wild plants must continuously evolve to fend off natural pathogens,points out RogerChetelat of the Tomato Genetics Resource Center at the University of California，Davis.But in their quest for size，shape and flavor，humans have inadvertently eliminateddefensive genes. As a result, most possess only a single disease-resistance gene.Perhaps that's the price to pay for a good, flavorful fruit? Hardly,Chetelat says, becausethe heirlooms' taste may have less to do with its genes than with the productivity of theplant and the growing environment. Any plant that sets only two fruits，as heirloomssometimes do, is bound to produce juicier,sweeter and more flavorful fruit than varietiesthat set 100, as commercial types do.*Plus, heirlooms are sold ripened on the vine,asurefire way to get tastier results than allowing them to mature on the shelf.

Now,Monsanto wants to do the same for the heirloom. In 1996 a tomato breeder andformer Tanksley student named Doug Heath began a pet project at SeminisVegetableSeeds, a Monsanto subsidiary. After 12 years of traditional breeding with thehelp of molecular markers,he has created a new rainbow-streaked tomato less prone tocracking and also endowed with 12 disease-resistant genes.The original plant，Heathexplains,had defective flowers,which is one reason why it set only two fruits comparedwith the 30 he gets from his new variety. He claims he is also able to maintain acomparable flavor and sugar profile even on productive plants. It tums out that theheirloom's defects are neither quirky nor cute, just an accident of a single-prongedbreeding strategy left over from the dawn of genetics.Heath's new plants may be availableto home gardeners next year and on commercial markets in the next three to five. "I seethem as coming to a pinnade," he says of the Rainbow and two other varieties he hasimproved with moderm stocks.But will heirloom adherents appreciate the look-alikes withhybrid seeds? "There will be a contingent of people," he says, "who will believe these are

poor imitations of the originals."

14-17為匹配題

14.F

15.E

16.c

17.A

19-22為填空EL

19.analysts

20.environment

21.salary

22. team

23-26為判斷題