The victim was last seen his office at 7 p.m.A. leaveB. leavesC. to leaveD. leaving

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A. leave
B. leaves
C. to leave
(答案→)D. leaving
解析：see sb doing sth表示看到某人正在做某事，根据题干提供的信息可知，the victim最后一次被看到离开是在早晨七点钟。表明在看到是，他正在离开，表示两个动作同时进行，故选D。