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Scientists SARS for several months before a major discovery was made in the fiel

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Scientists SARS for several months before a major discovery was made in the fiel

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

试题答案

A

标签：majorbeforewasdiscoverymonths

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