A、  B、  C、  C问题询问没告诉大家生日的原因(Why)，(C)项的回答“我没意识到”是符合题意的。以why开头的特殊疑问句是不能用Yes／No来回答的，所以(A)项被排除。(B)项中的party容易使人联想到题干中的

What’s the weather going to be like tomorrow?
(A) I think it’ll be nice.
(B) Tomorrow is the picnic.
(C) We’re going around two.

Why didn’t you tell us it was your birthday?
(A) Yes, it is.
(B) The party was great.
(C) I didn’t think about it.