他挥手示意公交车停下。

He waved his hand as a ___signal for the bus to stop.

收音机在山区接收不到清晰的信号。

The radio can't pick up a clear ___signal in the mountains.

他频繁看表的动作是一个信号，表明他很着急。

His frequent glances at his watch were a ___signal that he was in a hurry.

他举起手示意我们停下来。

He raised his hand to ___signal us to stop.