You must be careful when on the highway.A. you drivingB. are drivingC. drivingD. you were

问题补充：

You must be careful when on the highway.

A. you driving

B. are driving

C. driving

D. you were driving

答案：

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。