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
A. when was its full extent realized
B. was its full extent realized
C. that its fiill extent was realized
D. the realization of its full extent
Đáp án là B. cấu trúc đảo ngữ : Not until + clause/ adv of time + auxiliary + S + V