After working on the maths problem for hours,he found he thought was the key to

After working on the maths problem for hours,he found　 he thought was the key to it.

A.what　 B.that　 C.where　 D.which

试题答案

A

考查名词性从句。what引导的名词性从句作found的宾语从句的主语，而且what在名词性从句中作thought的宾语。其他选项不符合语法。