It was not until the beginning of the twentieth century to explore space.
A. did man begin
B. when man began
C. that man began
D. then man began
网友回答
A. did man begin
B. when man began
(答案→)C. that man began
D. then man began
解析：考查not until的强调句型。结构为：it is + not until+其它+that … ,故选C，句意为：就是直到二十世纪开始的时候人类开始探索太空。