交通信号灯是城市中常见的指示符号。

Traffic lights are common ___signifiers in urban areas.

在数学中，加号和减号是表示运算的重要符号。

In math, plus and minus are important ___signifiers for operations.

皮肤发红和发痒是过敏的常见标志。

Redness and itching are common ___signifiers of an allergic reaction.