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 extend 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： 直到肯塔基州的猛犸洞在1972年完全勘探完毕， 它的范围才全面被了解。not until结构位于句首时， 主句部分需要倒装。
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