caused the accident is still a complete mystery.A. WhatB. ThatC. HowD. Where

问题补充：

caused the accident is still a complete mystery.

A. What

B. That

C. How

D. Where

答案：

(答案→)A. What

B. That

C. How

D. Where

解析：本题考查名词性从句。从句中动词caused缺少主语，故用what作从句的主语。选A。