permanent signal

例句：

1. 1. The company's commitment to sustainability is a permanent feature of its operations.
   该公司对可持续性的承诺是其运营的一个永久性特征。
2. 2. The river flows through the town for centuries, a permanent reminder of its past.
   这条河流穿过这个城镇几个世纪，是一个永久的过去提醒。
3. 3. The invention of the printing press was a permanent change in the way information was disseminated.
   印刷术的发明是信息传播方式的一次永久性改变。