proves that Jack is really enthusiastic and cautious.
A. It happened that
B. When it happened
C. What happened
D. That has happened
网友回答
A. It happened that
B. When it happened
(答案→)C. What happened
D. That has happened
解析：考查主语从句。本题中的what引导起一个主语从句what happened，在句中做主语。句意：发生的事情证明Jack真的很热情很谨慎。故C正确。That在主语从句中不充当任何成分。A项意为碰巧。B项不能做主语。