It’s a sure sign that summer is over and winter is on the way — leaves that were once deep

It’s a sure sign that summer is over and winter is on the way — leaves that were once deep green turn red, yellow and bright orange. It happens every autumn. But did you ever wonder why?
The leaves start changing color when the nights start getting longer and the temperature starts getting cooler. Some scientists think that as the number of daylight hours shrinks, the leaves stop making chlorophyll which makes plants green. The reason is that the process of making chlorophyll requires sunlight. But according to horticulture (园艺) educator Susan Rose from Colorado State University, it turns out leaves don’t really change color at all. “The fall colors are actually there all along,” she said. “But they are covered by the green chlorophyll. As the chlorophyll stops being produced, the other colors can shine through.”
The kind of color that the leaves are going to change to is determined by the plant’s genetic (基因的) background. In some places, the leaves start changing color in September — before autumn even officially begins. In other places, they don’t change until late October or even November. Some scientists say the leaves have been changing color later than usual in recent years. And they think global warming has something to do with that.
In one study, researchers found that the growing season in the Northern Hemisphere was nearly a week longer, on average, in 2008 than it was in 1982. And a professor from Harvard University found that the leaves, west of Boston, Massachusetts, are changing color about three days later than they used to. However, other scientists disagree with the global warming theory. Susan Rose said the leaves in her part of Colorado are changing “right on schedule.” But the leaves in other parts of that state are running a little late. “The lateness in the mountains may have something to do with the really warm late summer and early fall,” she said. “But I’m afraid that’s just a guess.”
【小题1】According to Susan Rose, .A．the fall colors are always in leavesB．leaves make chlorophyll all the seasonsC．leaves’ changing color have nothing to with the weatherD．leaves begin changing color because they start making chlorophyll【小题2】The underlined word “shrinks” in Paragraph 2 could best be replaced by .A．reducesB．increasesC．lastsD．appears【小题3】Scientists guess leaves change color later than usual probably as a result of .A．genetic background B．dry weatherC．global warmingD．low temperature【小题4】The purpose of the last paragraph is to tell readers .A．why leaves change color at fall B．leaves change color at different timeC．the effects of the global warming on plantsD．different opinions on the lateness of leaves’ changing colorA

(答案→)A 解析：文章介绍秋天树叶变颜色的原因，还有现在树叶变绿的时间比以前迟了，科学家认为主要是因为全球气温变暖造成的，还有科学家给出不同的解释。【小题1】细节题:从第二段的句子：“The fall colors are actually there all along,” she said. “But they are covered by the green chlorophyll. 可知秋天的颜色一直在树叶里面。选A【小题2】猜词题：从后面的句子：the leaves stop making chlorophyll which makes plants green. 可知白天时间减少后，树叶就不再产生会使树变绿的chlorophyll ，选A【小题3】细节题：从第三段的句子：And they think global warming has something to do with that.可知科学家认为现在树叶变绿的时间比以前迟了，主要是因为全球气温变暖造成的。选C【小题4】细节题：从最后一段的句子：“The lateness in the mountains may have something to do with the really warm late summer and early fall,”可知最后一段是给出关于树叶变颜色推迟的不同解释，选D

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