open signal disc

例句：

1. 1. The engineer used an open signal disc to mark the location of the underground tunnel.
   工程师使用一个开放的信号盘来标记地下隧道的位置。
2. 2. The policeman used an open signal disc to warn other drivers of the upcoming construction work.
   警察使用一个开放的信号盘来警告其他驾驶员即将进行的建筑工作。
3. 3. The pilot used an open signal disc to communicate with the ground control tower during the flight.
   飞行员在飞行中使用一个开放的信号盘与地面控制塔进行通信。