Definition: to be quick to understand and react to things Level: C1
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.
