题目

题型：不详难度：来源：

Did you ever wonder why leaves change colors in autumn, or how it happens? Here’s a quick explanation.
First of all, you’ve probably noticed that not all trees lose their leaves. There are two major types of trees: deciduous trees and evergreens. Evergreens keep their leaves and stay green all year long. The deciduous trees are the ones that lose their leaves each year.
In autumn, the days begin to get shorter. The sun is also lower in the sky at noon, which causes temperatures to be cooler. The combination of less sunshine and lower temperatures is what causes the deciduous trees to drop their leaves. In winter, deciduous trees “go to sleep”, sort of like a bear goes into hibernation.
Leaves are the food factories for a plant. Unlike animals，plants make their own food by taking energy directly from the sun. Plants do this by using a chemical in their leaves called chlorophyll.
Chlorophyll is green. During the spring and summer the leaves of a deciduous tree look green because there is a lot of chlorophyll there. In autumn, however, the sunshine becomes more  scarce and the leaves on the trees stop making new chlorophyll. As the old chlorophyll starts to break down, the green color of the leaf begins to go away.
You’ve probably also noticed that the leaves of different kinds of trees change different colors. The leaves of some trees turn yellow or orange, while the leaves of other trees turn bright red or purple. The different colors are caused by the different chemical reactions that are happening in the leaves.
For example, the leaves of some trees turn yellow when the chlorophyll breaks down. When the green chlorophyll goes away, a yellow pigment in the leaf becomes visible. The yellow pigment, called cartenoid, was already there during the summer, but is was not easily seen because there was too much green chlorophyll.
In other trees where the leaves turn red or purple a different chemical reaction takes place. In those leaves the chlorophyll also breaks down, but another chemical reaction takes place at the same time. Chemicals in the leaves, called starches, begin to change into chemicals called anthocyanins, which give the leaves their red and purple colors.
So next time you see the beautiful colors of autumn you will also know the science behind that beauty.
46.   What is the correct term for trees that lose their leaves in the autumn?
A. Evergreens       B. Deciduous        C. Cartenoids  D. Anthocyanins
47.   Which of the following is NOT mentioned as a cause of trees dropping their leaves?
A. A change in temperature   B. The amount of daylight available
C. Extra chlorophyll         D. Chemical changes in the leaves
48.   If there are 50 evergreens and 10 deciduous trees in a forest, in autumn what color are most of the leaves in the forest likely to be?
A. GreenB. Red and purple
C. Yellow        D. not enough information to determine the answer
49.   Which of the following is only present in leaves during the autumn?
A. Chlorophyll       B. Starch C. Cartenoids        D. Anthocyanins
50.   Where would such an article mostly likely be published?
A. In a book                   B. In a primary school science text book
C. In a magazine for young people      D. All are equally likely

答案

 
46-50 BCADC  

解析

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