SINCE 1980, SCIENTISTS THE WORLD OVER _____ A LOT OF THINGS TO FIG...

Lớp 12 Tiếng Anh Bài tập tiếng anh lớp 12 theo chuyên đề

39. Since 1980, scientists the world over _____ a lot of things to fight against AIDS.A. have done B. are doing C. did D. had done