----- As we know you won’t go back to the USA since you have established a research cente

问题补充：

----- As we know, you won’t go back to the USA since you have established a research center here．But how long in America?

----- For almost 10 years．

A. did you study

B. have you studied

C. were you studying

D. have you been studying

答案：

(答案→)A. did you study

B. have you studied

C. were you studying

D. have you been studying

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