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It puzzles the scientists _____ some mammals produce their young _____ others lay eggs.
A.that; while B.what; whileC.that; as D.what; as
A

试题分析:本句中的it是形式主语 ,真正的主语是后面that引导的主语从句,that在主语从句中不充当任何成分,只起引导作用。第二空的连词while然而,表示上下文轻微的转折关系。As不能表示上下文的转折关系。句义:让一些科学家困惑的是一些哺乳动物生出下一代,而另外一些却产卵。故A正确。
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