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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

試題分析：本句中的it是形式主語 ，真正的主語是后面that引導的主語從句，that在主語從句中不充當任何成分，只起引導作用。第二空的連詞while然而，表示上下文輕微的轉折關系。As不能表示上下文的轉折關系。句義：讓一些科學家困惑的是一些哺乳動物生出下一代，而另外一些卻產卵。故A正確。
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