frequency marker

例句：

1. 1. The frequencyof earthquakes in this area is relativelyhigh.
   这个地区的地震频率相对较高。
2. 2. Thefrequency of occurrence of this event is low.
   这种事件的发生频率很低。
3. 3. Thefrequency of the signal changeswith time.
   信号的频率随时间变化。