( )is the most unstable element of a language as it is undergoin

( )is the most unstable element of a language as it is undergoing constant changes both in form and content.
【正确答案】：VOCABULARY
【题目解析】：词的形式和内容总是处于变化之中，是语言中最不稳定的因素。不过相比之下，词的内容比形式更不稳定。