Question

 (2011年陜西西安分科質檢)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：選A。what引導的名詞性從句作found的賓語從句的主語，而且what在賓語從句中作主語。he thought為插入語。