when did you get home last Friday?It was not until midnight（

选A when
若是B项,答语是强调句,把多余部分去掉就是：it was not raining hard until midnight.答语是在强调午夜雨很大,其重点不是在回答when.
而A项,答语中when引导了一个定语从句,整个句子关键词是 not until midnight ,正好回答了问句询问的时间.
故A为正解~