尽管年事已高，他在科技方面仍然反应敏捷。

Despite his age, he ___is ___on ___the ___ball when it comes to technology.

在繁忙的机场，你必须保持机敏才能赶上航班。

In busy airports, you must ___be ___on ___the ___ball to catch your flight.

为了胜任这份工作，你必须时刻保持警觉。

To succeed in this job, you have to ___be ___on ___the ___ball at all times.