Question

 (2011年陕西西安分科质检)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

Answer 1

解析：选A。what引导的名词性从句作found的宾语从句的主语，而且what在宾语从句中作主语。he thought为插入语。