A．the Indo­Pacific has the largest coral coverage

　　B．reefs only began to disappear in the 1960s

　　C．coral loss has only happened in some parts of the world

　　D．human factors play an important role in coral loss

　　解析　推理判断题。从文章第一段的介绍可知，珊瑚礁的消失与三个因素有关，而气候变暖和海岸开发都有人为因素在里面。

　　答案　D

　　42．In what way are coral reefs important?

　　A．Many marine plants and animals

　　live in them.

　　B．They can be used as building materials.

　　C．Many marine plants and animals feed on them.

　　D．They provide shelter for island residents.

　　解析　细节理解题。文章倒数第二段第一句说明，珊瑚礁为很多海洋动植物提供了“家”。

　　答案　A

　　C

　　How much rain has fallen on the earth in the past？Man has not always kept weather records.Because scientists need a way to learn about past rainfall，they study the tree rings.

　　A tree's trunk(树干)grows bigger each year.Under its bark(树皮)，a tree adds a layer of new wood each year it lives.If you look at a tree stump(树桩)，you can see the layers which are called annual rings.

　　On some trees，all of the rings are of the same width.But the ponderosa pine(美国黄松)that grows in the American southwest have rings in different width.The soil in the southwest is dry.The pines depend on rainfall for water.In a year of good

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