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

10/80

Mark the letter A, B, c or D on your ansyver sheet to indicate the correct answer which completes each of the following sentences
________ Not until Kentucky’s Mammoth Cave had been completely explored in 1972

when was its full extent realized

was its full extent realized

that its fiill extent was realized

the realization of its full extent

Giải thích

Đáp án là B. cấu trúc đảo ngữ : Not until + clause/ adv of time + auxiliary + S + V