题目内容：
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
参考答案：
选择题：Much research _______ into the possible causes of
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- 题目类型：选择题
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(A) has been done
(B) was done
(C) have been done
(D) were done