单选题
Not until Kentucky's Mammoth Cave had been completely explored in 1972______.
A、
when was its full extent realized
B、
that its full extent realized
C、
was its full extent realized
D、
the realimtion of its full extent
【正确答案】
C
【答案解析】
not until结构位于句首时，主句部分需要倒装，选项C正确。
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