求大神解释一下ofwhichg和only4%为什么能互换？最好帮解释一下句子架构成分！谢谢！！

更新时间2018-07-16 15:33:10

70% of the earth surface is covered with water, only 4% of which is fresh water. 70% of the earth surface is covered with water, of which only 4% is fresh water.

of which大概是可以忽略不计的吧所以它放在哪里没关系就是一个从句而已考试应该不会考这么细吧理解意思就行

70% of the earth surface（主语） is（谓语） covered with water（过去分词做后置定语表被动）, only 4% of which is fresh water.（从句）