You must be careful when on the highway.
A. you driving
B. are driving
C. driving
D. you were driving
网友回答
A. you driving
B. are driving
(答案→)C. driving
D. you were driving
解析：考查状语从句的省略。当状语从句的主语与主句的主语一致，且状语从句中有系动词be的时候，可以把状语从句的主语和be动词一起省略。本题在when后面省略了you are；句意：当你在高速公路上开车的时候，你一定要很细心。故C正确。