—How dangerous it was!—Yes, but for the passer-by’s quick action, the girl .A. was drowned

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A. was drowned
(答案→)B. could have been drowned
C. had drowned
D. should be drowned
解析：考查情态动词+have done的用法，But for是要不是，表示和过去事实相反的情况，主句是could have done表示和过去情况相反的情况，the girl和drown是被动关系，句意：--多么危险啊！--是的，要不是过路人的快速反应，女孩就会被淹死了。选B。