These projects have proved to be very effective in dealing with global climate change, which was considered an insoluble problem in the first 30 years of this century.

These projects have proved to be very effective in dealing with global climate change, which was considered an insoluble problem in the first 30 years of this century.

A. hard

B. challenging

C. unsolvable

D. unbearable
Xem đáp án
C là đáp án đúng
Lời giải:
insoluble = unsolvable (không thể giải quyết)