Much research _______ into the possible causes of the disease in the past few years.<br> (A) has been done<br> (B) was done<br> (C) have been done<br> (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：新疆工程学院大学英语1

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