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The suspected man was seen ______ the building.
A、enter
B、entering
C、entered
D、to enter
【正确答案】：D
【题目解析】：本题考查被动语态。有些动词后跟不带to的不定式作宾语补足语，但改为被动结构后要加上“to"。See sb.do sth.改为被动式后应该为：sb.be seen to do sth.。这类词有see,hear,ask等。故本题选D。答案为D。