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2020托福启航班结课测试（仅限电脑，请勿用手机参与）

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In the late 1990s, the effects of the bark beetle epidemicrippled throughout Alaskas white spruce ecosystem and affected virtually every population of living organisms, but not all of the impacts were negative. Fewer spruce trees meant a sunnier area in the forest below the treetops, which allowed grasses to move in and take hold. The grasses, in turn, changed the soil temperature, making the environment more friendly for some other types of vegetation. Animals that feed on grasses, including moose, elk, and some birds, also benefited. But the beetle infestation was bad news for organisms that rely on white spruce for their habitat, like hawks, owls, red squirrels, and voles. Voles a type of small, mouselike rodent are an especially vital part of the ecosystem because they help spread mycorrhizal fungi, which attach to the roots of plants and help them take in water and nutrients. Voles are also an important food for a number ofpredators.

According to paragraph 3, a decline in the vole population in Alaska may have which TWO of the following consequences？To receive credit, you must select TWO answer choices.

A.Some predators may have less to eat.

B.Hawk and red squirrel populations may be more successful.

C.Plants may find it more difficult to absorb water and nutrients.

D.Mycorrhizal fungi numbers may increase.

█Ecosystem changes always hurt some living creatures and help others. Its hard to say,therefore, whether a change is good or bad overall. █Instead, ecologists (people who study ecosystems) often focus on the impacts on a single species: for instance, us. █ In the short term, the Alaskan spruce beetle epidemic supplied a lot of people with firewood, but only by destroying tons of otherwise valuable timber and threatening the livelihoods of loggers. █And no one knows for sure what the long-term impacts on the forest will be. Ecosystems tend to return totheir previous states after disturbances like pest outbreaks, fires, or major storm events,but if the Alaskan spruce ecosystem is disturbed too often or too much, it might shiftto a different type of forest, a woodland, or a grasslandinstead.

What does the professor mainly discuss?

A.The popularity of city scenes in nineteenth-century paintings

B.A popular nineteenth-century art form that is little known today

C.Techniques developed by nineteenth-century artists to make their work look realistic

D.Reasons that art became a popular form of entertainment in the nineteenth century

According to the professor, why were so many spectators interested in seeing a panorama titled a view of Paris from the roof of the Tuileries?

A.It was a rare opportunity for London residents to see a realistic depiction of Paris

B.It was the first time that an artist had created a painting while working on the roof of a high-rise building

C.They had heard that it offered a view of Paris that no one had ever seen before

D.They knew that they would be able to confirm the accuracy of the paintings details for themselves

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