On his way home yesterday , Tom was hit on the head he had time to realize what was happening .
A. since
B. before
C. when
D. until
网友回答
A. since
(答案→)B. before
C. when
D. until
解析：考查时间状语从句。句意“昨天在回家的路上，汤姆还没意识到发生了什么事，就被击中了头部。”before“在......之前，还没来得及......”。故选B。