A
Some trees drop their leaves every year. Their leaves grow back again in spring.
Scientists think that plants don't want things they can't use. When leaves lose their chlorophyll(叶绿素), they fall off.
A leaf's chlorophyll uses sunlight to make sugar out of water and carbon dioxide(二氧化碳). Plants need carbon dioxide to live and grow. When leaves use carbon dioxide, another gas called oxygen is left over. The plant can't use the oxygen, so it lets the oxygen go.
Animals and humans need oxygen to live. Their bodies use the oxygen, and what do you think is left over? Carbon dioxide. When animals and humans breathe out, they let the carbon dioxide go. It's easy to see that plants, animals and humans help each other in this way.
In places where it gets cool in autumn, a plant loses its chlorophyll, and its leaves may turn yellow or red. Lutein(叶黄素), which makes the leaves yellow, was there all summer, but there was so much chlorophyll that the yellow didn't show until the green went away. Yellow leaves turn red only if they have lots of sugar in them and the sunlight shines on them. The more sugar a leaf has, the redder it becomes. If a leaf is kept in the shade(阴凉处), it will stay yellow even if it has a lot of sugar.
()1. Which of the following has no effect on sugar production according to the text?
A. Chlorophyll.
B. Sunlight.
C. Water.
D. Oxygen.
()2. What stops yellow colour from being shown?
A. Chlorophyll.
B. The temperature.
C. The height.
D. The weight.
()3. Where is the text most probably from?
A. A science magazine.
B. A storybook.
C. A travel guide.
D. A dictionary.
()4. What's the best title for the text?
A. Effect of Sugar on Leaves
B. Carbon Dioxide and Oxygen
C. The Importance of Sunlight
D. The Secret of Leaves Changing Colours

A 1—4 DAAD