(2013·安徽，21)From space, the earth looks blue.This is ________about seventyone percent of its surface is covered by water.
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why
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What makes online shopping so popular is it shortens the distance between manufacturers and consumers．
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how
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that
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What is concerning us greatly is ____ the workers held up in that area yesterday will be set free.
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Courage is a good quality and is ________ it takes to get over difficulties.
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Mary is a humorous and warmhearted woman and that is ________ she is a most popular person in her community.
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