You are too late. The film since half an hour ago. A. has begun B. has been onC. beganD.

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A. has begun
(答案→)B. has been on
C. began
D. begins
解析：考查时态。句意“你太晚了。电影已经开始半小时了。”since从句表示一段时间，前面用现在完成时，begin是短暂性动词，不能与一段时间连用，需要转换为be on。故选B。