They to enter the building by the back door; however the front door was locked.A. are su

问题补充：

They to enter the building by the back door; however, the front door was locked.

A. are supposed

B. aren’t supposed

C. weren’t supposed

D. were supposed

答案：

A. are supposed

B. aren’t supposed

(答案→)C. weren’t supposed

D. were supposed

解析：考查短语：be supposed to do 应该做某事，根据后半句the front door was locked.，可知前半句也应用过去式，句意：他们不应该通过后门进入这个大楼，可是，前门锁上了。根据句意应选C。