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  3. It is a common belief that sharp frosts color the autumn leaves. As a matter of fact, such frosts are far more likely to turn le

It is a common belief that sharp frosts color the autumn leaves. As a matter of fact, such frosts are far more likely to turn le

admin2020-04-14  70
问题     It is a common belief that sharp frosts color the autumn leaves. As a matter of fact, such frosts are far more likely to turn leaves black or dull brown than to give them the gorgeous tints we admire.
    The coloring of the leaves in the fall is a chemical process that is favored by gradual cooling rather than sudden cold. It is not entirely confined to the autumn. Bright red and yellow leaves are often found on the swamp maple and other trees, even in summer.
    Through the season of growth the leaves serve as food factories for the trees. In their tiny cells the carbon of the air is combined with materials brought up by the tree fluid from the roots to form the starch, sugar, and other substances by which the whole tree is fed. The food-making process is performed by sunshine with the aid of a substance called chlorophyll. Chlorophyll is a mixture of several pigments, or coloring matters. One of these is green, and gives the leaves their ordinary color. Another is yellow and is the same substance that, on account of its abundance in growing grass, makes butter particularly yellow in the spring.
    When the cool weather sets in and the growth of vegetation slows down, the trees need less food and gradually suspend work in the leaf factories. Both the food and the chlorophyll in the leaves are drawn into the body of the tree and stored up for use in the spring. This transfer involves many chemical changes. One of them is the breaking up of the chlorophyll into the substances of which it is composed. The green pigment passes out of the leaves before the yellow. Thus yellow becomes one of the prevailing hues of the autumn foliage.
    The reds, which also prevail in the autumn, do not come from the chlorophyll, but from pigments contained in the sap. Their appearance indicates an excess of sugar in the leaves, after the withdrawal of other materials. It is supposed, also, that the reddening of the leaves protects the food materials from the harmful effects of strong light during their passage into the tree. The same red coloring is seen in the buds of many plants in the spring, where it probably also serves a protective purpose.
Yellow prevails in the autumn leaves mainly because ______.
选项 A、the food-making process ceases when it becomes cool
B、chlorophyll breaks up into various pigments in cool weather
C、the yellow pigment remains in the leaves longer than the green
D、the transfer of chlorophyll involves complicated chemical changes
答案C
解析 第4段末句开头的thus表明该句与前一句构成因果关系，选项C是对第4段倒数第2句的近义改写，为本题答案。
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