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  • 更新 2022-09-03
  • 科目 英语
  • 题型 单选题
  • 难度 较易
  • 浏览 1187

Scientists ______ SARS for several months before a major discovery was made in the field.

A.had been studying B.have been studying
C.have studied D.are studying
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ScientistsSARSforseveralmonths