摘要： The Chinese government research into vaccine against H1N1for months, recently many have received the first doses. A. is doing B. has done C. had been doing D. did
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The Chinese government ________ research into vaccine against H1N1for months, recently many have received the first doses．
- A.is doing
- B.has done
- C.had been doing
- D.did
has done B. did C. was doing D. had been doing did