By the year 2020 scientists a lot of research on AIDS.A. had doneB. doC. are doingD. will

发布时间:2020-08-16 08:17:04


By the year 2020 scientists      a lot of research on AIDS.
A. had done
B. do
C. are doing
D. will have done

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A. had done
B. do
C. are doing
(答案→)D. will have done
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