单选题 Richard came home at midnight, ______ which time all the guests had left.
【正确答案】 B
【答案解析】[解析] 句意：理查德午夜回家时，所有的客人已经离开了。by which time定语从句，“到那时”。at midnight是一个时间点，during“在…期间”不符合题意。