老师发出信号后，孩子们都举起了旗子。

At a ___signal from their teacher, the children all held up their flags.

在我发出信号之前不要动。

Don't move until I give the ___signal.

比赛开始的信号通常是鸣枪。

The ___signal for a race to start is often the firing of a gun.