下面这句话的from which的先行词是什么?it指代什么?

问题描述:

下面这句话的from which的先行词是什么?it指代什么?
A constituent of a mixture,substance,or system is one of the things from which it is formed.

这是单词constituent的一个例句,以示意它有"成分"的意思.
which指的是constituent,
it指的是"a mixture,substance,or system".对,我是看到这个单词constituent的例句解释,然后看到这个不是很明朗。which 的先行词是constituent?如果which 的先行词是constituent,是不是该这么说:A constituent of a mixture, substance, or system from which it is formed is one of the things .which的先行词在语法上是one of the things, 从句子意思的角度讲它们都是指被解释的词constituent.