In the second sentence of the last paragraph the word "they" refers__________.

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In the second sentence of the last paragraph, the word "they" refers__________. 查看材料

A.Scientists engaged in the experiments

B.Investigations made by people

C.The resistant strain and the sensitive strain

D.The availability of the two strains

请帮忙给出正确答案和分析,谢谢!

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