Ifhumanbeingshadbeenabitlessgreedyandcruel,morebirdsandanimals_______dyingout.
A.ought to avoidB.could have been avoidedC.should have avoidedD.might have avoided在线课程D

本题涉及虚拟语气的使用，从if引导的从句分析可以知道与过去相反，should have done表示本来不应该但实际上干了，不符合题意。