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

发布时间：2020-08-17 10:14:56

     caused the accident is still a plete mystery.
A. What
B. That
C. How
D. Where

网友回答

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。
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