一只年轻的狗应该机警而活泼。

A young dog should be ___alert and playful.

教师需要对学生行为的突然变化保持警觉。

Teachers need to be ___alert to sudden changes in students' behavior.

一点紧张感没关系——我想它让你在开车时保持警觉。

A little bit of tension is alright - I guess it keeps you ___alert while driving.