阅读理解

阅读下列短文，从每题所给四个选项(A，B，C和D)中，选出最佳选项．

　　Plants need green leaves to make food. A plant needs sunlight and carbon dioxide (二氧化碳) from the air for making food and it also needs water and salts from the soil to make food too. There are certain cells(细胞) in the leaves which change carbon dioxide and water into sugar. To do this the cells need energy, which they get from the sunlight.

　　Green leaves make food for the whole plant. A red leaf can make food too because under the red coloring of the leaf there are food-making cells. There are no leaves which are completely yellow, for they can't make food.

　　The plant makes sugar for its food. In sunlight green leaves make a lot of sugar. The veins (叶脉) can't carry all this sugar away, so the leaves change the sugar into starch(淀粉), which is kept and so stored in the leaves. At night the starch change back to sugar. It is then carried away from the leaves. Some of the sugar is used as food by the plant while the rest is stored as starch. In some plants, food is stored in the roots. In others it is stored in the stem and in leaves, fruits and seeds.

1．Which of the following best gives the main idea of this passage?

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A．How green plants make food.

B．How green plants make sugar.

C．How green plants get their energy.

D．How green plants changes sugar into starch.

2．Sugar is made for its food by ________.

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3．Food is stored in roots or stems in the form of ________.

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