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

—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

试题答案

A

解析:

略