Capital F, s: find the s that's before that.

大写 F, s：找到前面的 s。

But when planes came along, radio communication was more common and when spoken, SS can easily be confused with the letter F S O S. What did you say?

但当飞机出现后，无线电通信变得更加普遍，在说话时，SS 很容易与字母 F S O S 混淆。你说什么？