单选题
Not until Kentucky’ s Mammoth Cave had been completely explored in 1972 _____.无
A、
when was its full extent realized
B、
that its full extent was realized
C、
was its full extent realized
D、
the realization of its full extent
【正确答案】
C
【答案解析】
句意： 直到1972年肯塔基州的猛犸象洞被完整探索后， 洞内的全部内容才被发现。 Not until置于句首时， 主句应倒装。 未倒装前的结构为“It is not until. . . that. . . ” 。 因此， 本题的正确答案为C。
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