SINCE 1980, SCIENTISTS IN THE WORLD ________A LOT OF THINGS TO FIGH...
16.
Since 1980, scientists in the world ________a lot of things to fight AIDS.
A. have done
B. are doing
C. did
D. had done
16.
Since 1980, scientists in the world ________a lot of things to fight AIDS.
A. have done
B. are doing
C. did
D. had done