By the time human beings become fully aware of the importance of the ozone layer, it ______

Câu hỏi :

Mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions

A. will be

B. will have been

C. is

D. has been

* Đáp án

B

* Hướng dẫn giải

Đáp án B

Dấu hiệu nhận biết thì hoàn thành “By the time…”

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