Mark the letter A, B, C, or D to indicate the word(s) CLOSEST in meaning to the underlined word(s) in each of the following questions.
Question. 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
Đáp án C
Insoluble = unsolvable (không thể giải quyết)