她已年过九十，但仍然反应敏捷。

She is over ninety, but she ___is ___on ___the ___ball.

旅行时，你应该时刻保持警觉。

When traveling, you should ___be ___on ___the ___ball at all times.

在谈判中，保持机敏至关重要。

In negotiations, it's crucial to ___be ___on ___the ___ball.