SINCE 1980, SCIENTIST THE WORLD OVER _____ A LOT OF THINGS TO FLIGHT AGAINST AIDS

7. - TONG HOP BAI TAP VE TU LUAN TONG HOP BAI TAP VE TU LUAN
TONG HOP BAI TAP VE TU LUAN TONG HOP BAI TAP VE TU LUAN

7.

Since 1980, scientist the world over _____ a lot of things to flight against AIDS.

A. have done

B. are doing

C. did

D. had done