Hejust before the concert started.A. dropped outB. dropped awayC. dropped inD. dropped up

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(答案→)A. dropped out
B. dropped away
C. dropped in
D. dropped up
解析：短语辨析。Drop out退学，脱离；drop away离开，逐渐减少；drop  in 落入，逐渐进入；drop up不期来到。句意：就在音乐会开始前他退出了。故选A。