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

问题补充：

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

答案：

(答案→)A. had been studying

B. have been studying

C. have studied

D. are studying

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