Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions.
Not until Kentucky’s Mammoth Cave had been completely explored in 1972_________
A. when was its full extent realized
B. the realization of its full extent
C. was its full extent realized
D. that its full extent was realized
Đáp án C
– cấu trúc Not until + N/mệnh đề, mệnh đề đảo: không cho đến tận khi.
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