It puzzles the scientists some mammals produce their young others lay eggs.
A. that; while
B. what; while
C. that; as
D. what; as
网友回答
(答案→)A. that; while
B. what; while
C. 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。