问题
选择题
. By the year 2020 scientists _____ a lot of research on AIDS.
A．had done
B．do
C．are doing
D．will have done
答案
答案：D
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. By the year 2020 scientists _____ a lot of research on AIDS.
A．had done
B．do
C．are doing
D．will have done
答案：D