问题描述：
英语 求详细解析
Research___all over the world into the possible cause of cancer in the past years.
A.was made B. had been made C.has been making D.has been made
可以理解为什么用完成时态,但是,怎么理解这句话?into做为什么的连接介词啊?
问题解答：
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