SIGN function

例句：

1. 1. The policeman raised his hand as a sign to stop the car.
   警察举手示意停车。
2. 2. The new policy is a clear sign of progress.
   新政策是进步的明显标志。
3. 3. The flag was flown at half-mast as a sign of mourning.
   旗帜降半旗表示哀悼。