undersea mountains generally form when volcanic mountains rise up from the seafloor but fail to reac

题目

undersea mountains generally form when volcanic mountains rise up from the seafloor but fail to reach the surface,while (those that) break the surface become islands.
这里that有什么意义?
定语从句that which一般情况可以省略吗?什么时候不能省略?

答案

undersea mountains（主语） generally form（谓语） when volcanic mountains rise up from the seafloor but fail to reach the surface时间状语while (those《those 指代volcanic mountains 》 that break the su...