The number of historic buildings which has reached a surprising figure that we must be aw

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A. has been pulled down
B. had been pulled down
C. is being pulled down
(答案→)D. have been pulled down
解析：选D　考查时态和主谓一致。句意：我们必须意识到，已被拆毁的历史建筑的数量惊人。主语部分含有一个which引导的定语从句，先行词是buildings，因此从句中的谓语应该用复数形式。再根据句意可知，此处用现在完成时，表示“已经被拆毁的”。