Climb mountains中mountain为什么要加s

mountains是指山，山脉，高山等意思，climb mountains是爬山的意思，mountains要加s是规定的用法，例如：She has a desire of climb mountains.她想去爬山。

This town is beset with towering mountains.这座城市为高山所环抱。Did you see those mountains 你看到那些高山了吗？