C 这是not...until结构的强调句型.
其结构为:It be not until+时间状语(从句)+that+(主)句的肯定形式
如:I won't go to bed until 11:30 every evening.
=It is not until 11:30 every evening that I'll go to bed.
I didn't go to bed until she returned last night.
=It was not until she returned that I went to bed.