I worked

I worked __3years ago.A.in hospital B.at hospital C.at a hospital
以上三个答案有何区别?

正确答案：I worked at a hospital 3 years ago.
其他的没有了a 语法上不对:
I worked in a hospital,besides working on TV.
复数：I worked in hospitals,besides working on TV.
I worked at/in a hospital 3 years ago.
下面句子里重要的部分是patient+地方所以不用a ：
Sometimes,the health of a patient in hospital may get worse suddenly.
My aunt was put in hospital after seizure.