Not until Kentucky’s Mammoth Cave had been completely explored in 1972

Câu hỏi :

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

* Hướng dẫn giải

Đáp án C

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