A signal feat的用法和样例：
- A signal feat.
- A red light is usually a signal of danger.
- The rise in prices was a signal for rebellion.
- Her speech yesterday was a signal that her views have changed.
- I'll cue you in (ie give you a signal to start) by nodding my head.
- I'll say `One, two, three, go!' as a signal for you to start.
我喊｀一、 二、 三、 开始!＇你就开始。