Mary couldn✀t have received your letter;otherwise she___ to your birthday party.

答案：C.

翻译：玛丽不可能收到了你的信，否则她早就会来参加你的生日晚会了。

解释：此题考查含蓄的虚拟语气。
1. 由前句的couldn't have...可知“事实上玛丽并没有收到你的信”，再根据后句的otherwise，所以后句的实际情况就是“玛丽也就没有来参加你的生日晚会”。
2. 由此可以推断，后句是对过去事实的虚拟。
可以做如下推理：
otherwise she___ to your birthday party
= If Mary had received your letter, she would have come to your birthday party.
对过去事实的虚拟，主句谓语由“would/could/should/might have done sth”构成，所以正确答案为C。

C reason:这个问题考查的是虚拟语气（对过去虚拟）,你可以记住一个固定模式，以后类似的题目你就可以自己解决了------somebody would/could/might have done something if somebody had done something!!!
could'nt have done something----不可能做过某事