It puzzles the scientists some mammals produce their young others lay eggs. A. that; wh

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

(答案→)A. that; while B. what; whileC. that; as D. what; as解析：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。

感谢回答，我学习了