答案B 时间状语从句
It was midnight when____ he arrived home
不能选择A
因为 它不是强调句
需要改为
It was at midnight __that__ he arrived home 怎样区分强调句和状语从句强调句就是 去掉it is （was） 。。that。。句子成分还是完整的It wasatmidnight __that__ he arrived home 就拿这个来说 去掉 it was thathe arrived home at midnight 完整 但是It was midnight ____ he arrived home如果这里填上that 去掉it was 。。that 之后 就变为he arrived home midnight是错的 时间名词前 缺少介词 不能构成完整的句子 所以就填when 做时间状语从句