3273In the second sentence of the last paragraph， the word ”they” refers__________． 7169

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！