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.
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.