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题目内容
（南阳师范学院-教育学-大学英语（四））
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
(A) has been done
(B) was done
(C) have been done
(D) were done
参考答案