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

问题补充：

caused the accident is still a complete mystery.

A. What

B. That

C. How

D. Where

答案：

A. What

B. That

C. How

D. Where

解析：A

解析主语从句中缺少主语，而能在主语从句中既充当主语又起引导作用的只有what。