It was not until dark__he worked out the last result of the experiment.A what B that C when D before为什么不能选C而选B?

问题描述:

It was not until dark__he worked out the last result of the experiment.
A what
B that
C when
D before
为什么不能选C而选B?

not until 已经是完整的时间陈述短语
it is that 是强调这个时间段