问题补充：
He didn’t attend the meeting last night, he doesn’t know our plan for the picnic.
A. that’s why
B. that’s because
C. which is why
D. which is because
答案：
A. that’s why
B. that’s because
(答案→)C. which is why
D. which is because
解析：句意：他昨晚没有出席这个会议，那就是为什么他不知道我们野炊的计划。which 引导非限制性定语从句，而that是不能引导非限制性定语从句的，根据句意，故选C
