必须保持

On this superhighway drivers should keep to a speed of over 80 miles per hour.

在这条高速公路上,驾驶员必须每小时80英里以上的速度。

A policeman has to keep on his toes all the time he's on duty.

警察在执勤时必须时警覺。

Athletes have to train continuously to stay in peak condition.

运员必须练才能最佳竞技状态。