signal indication checking

例句：

1. 1. The traffic light is acommon signal for pedestrians crossing the street.
   交通灯是行人过马路的常见信号。
2. 2. The policeman used his hand asa signal to stop the car.
   警察用手势示意停车。
3. 3. The doctor gave her athumbs-up as a signal ofapproval.
   医生向她竖起大拇指表示赞同。