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  3. Many trees and shrubs change color in fall. Three factors influence fall’s colorful farewell-leaf pigments (色素), length of night

Many trees and shrubs change color in fall. Three factors influence fall’s colorful farewell-leaf pigments (色素), length of night

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问题     Many trees and shrubs change color in fall. Three factors influence fall’s colorful farewell-leaf pigments (色素), length of night, and weather. The timing of the color change is mainly【S1】______by the increasing length of night. None of the other【S2】______influences, such as temperature, rainfall, food supply, are as unchanging as the【S3】______increasing length of night during fall. As days grow shorter, and nights grow longer and cooler, biochemical processes in leaves begin to【S4】______the landscape with an explosion of colors. And nature puts on one of its most wonderful displays of beauty.
    The timing of the color change varies by【S5】______. Some species in southern forests can become vividly colorful in late summer while all other species are still vigorously green. Oaks put on their colors long after other species have already【S6】______their leaves. These differences in timing among species seem to be【S7】______inherited, for a particular species, whether on a high mountain, or in warmer lowlands, will change color at the same time.
    However, some species are【S8】______. Pines, for example, are green all the year round because they have toughened up. They have developed over the years a needle-like leaf, which is【S9】______with a heavy wax (蜡状的) coating. And the liquid inside their cells contains cold-resistant elements. So the leaves of evergreen can safely withstand all but the most【S10】______winter conditions, such as those in the Arctic.

A) severe              E) species              I) shed                  M) covered
B) paint               F) regulated            J) steadily              N) attractive
C) genetically         G) enthusiastic         K) evergreens            O) faded
D) present             H) environmental        L) present
【S6】
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答案I
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大学英语四级

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