必须保持

Athletes have to train continuously to stay in peak condition.

运动必须不断训练才能保持最佳竞技状态。

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

警察在执勤时必须时刻保持警覺。

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

在这条高路,必须保持每小时80英里以的度。