Much research _______ into the possible causes of the disease in the past few years.　 (A) has been done (B) was done (C) have been done (D) were done

Much research _______ into the possible causes of the disease in the past few years.　
(A) has been done
(B) was done
(C) have been done
(D) were done

出自：南阳师范学院-教育学-大学英语（四）

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