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(yǔ) ，真正的主語(yǔ)是后面that引導(dǎo)的主語(yǔ)從句，that在主語(yǔ)從句中不充當(dāng)任何成分，只起引導(dǎo)作用。第二空的連詞while然而，表示上下文輕微的轉(zhuǎn)折關(guān)系。As不能表示上下文的轉(zhuǎn)折關(guān)系。句義：讓一些科學(xué)家困惑的是一些哺乳動(dòng)物生出下一代，而另外一些卻產(chǎn)卵。故A正確。
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