advertising is a form of communication used to help sell products and services.是什么从句啊,谢
问题描述：
advertising is a form of communication used to help sell products and services.是什么从句啊,谢
那是不是省略了which was呢，如果是定语从句的话，used 前面是不是省略了which was。
答
定语从句
省略的是which,而不是which was,因为be used to “习惯...”的意思