Passage Three Questions 21 to 25 are based on the following passage.
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(颜色)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 (叶绿素). Chlorophyll is a mixture of several pigments(色素) , 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[ap] in the spring . When the cool weather sets in and the growth of 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 form the harmful effects[ap] 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.
21. According to the passage , which of the following is true about the coloring of leaves? A) Sharp frosts color the autumn leaves. B) The coloring of leaves in confined to autumn. C) The coloring of leaves is a chemical process. D) In the fall , leaves turn to a black or dull color.
22. Butter looks yellow in the spring because _______. A) spring is the season of growth B) the substance of yellow in chlorophyll is in great quantity C) sunshine has a strong effect on it D) there is no green in its chlorophyll
23. When the cooling process begins, _______. A) chlorophyll leaves the leaves B) red becomes dominant in chlorophyll C) sunshine is not sufficient[ap] to support the food making process D) food and chlorophyll are kept in the body of the tree for use in spring
24. The red coloring protects _______. A) chlorophyll from losing B) the buds of many plants C) food materials from strong light D) pigments in the sap
25. Which of the following may best serve as the title? A) The Chemical Changes Inside a Tree . B) The Colors of Leaves . C) How Autumn Leaves Get Their Colors. D) The Gorgeous Tint of Autumn.
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.
