Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions
By the time human beings become fully aware of the importance of the ozone layer, it ______ seriously damaged
A. will be
B. will have been
C. is
D. has been
Đáp án B
Dấu hiệu nhận biết thì hoàn thành “By the time…”