随机的审计让每个人都保持警觉。

Random audits ___keep everybody ___on ___their ___toes.

教练不断变换战术，让对手保持警惕。

The coach constantly changes tactics to ___keep the opposition ___on ___their ___toes.

随堂测验让学生们在整个学期中都保持专注。

Surprise quizzes ___keep students ___on their ___toes throughout the term.