They ________ to enter the building by the back door; however, the front door was lockedA.are supposedB.aren’t supposedC.weren’t supposedD.were supposed
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C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。