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
网友回答
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。