A、  B、  C、  C问句以疑问词why开头，询问某人没有参加聚会的原因。(C)回答说是因为太忙，所以是正确答案。(A)是关于时间的回答，因此是错误的。

We need to start immediately, don’t we?
(A) Yes, if we want to be on time.
(B) Twenty minutes, I suppose.
(C) Work starts at 9 o’clock, usually.

Why hasn’t he come to the gathering?
(A) Not until tomorrow, I guess.
(B) I know, he should have called.
(C) He said he was busy.