抛掷两枚质地均匀的硬币，已知有一枚出现“正面向上”，那么另一枚出现“正面向上”概率是______．

— Why didn't you come to our party yesterday? There was a lot of fun.

— I _________, but I had an unexpected guest at home.

A. had

B. was going to

C. did

D. would