问题
选择题
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
答案：C
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。