Our teacher insisted that the key words worth paying attenti

选C；to be underlined
Our teacher insisted that the key words worth paying attention (to should be underlined )before class.
pay attention to的宾语是key words,
the key words (should be underlined )before class.在insist 引导的从句中用虚拟语气should省略；所以就剩下C项中的to是介词而不是不定式符号,A项缺 介词to ;如果选B和D项没有谓语动词；
所以选C.

our teacher insisted that the keys words worth paying attention to be underlined.
pay attention to do sth.
这里要用 to be done , 因为，宾语从句中的主语words 与动词谓语underline 属于被动关系。

C to be underlined
pay attention to do sth
our teacher insisted that the key words worth paying attention to be underlined before class.
老师强调说 我们在课前就应该把中心词着重的划下来
key words 做主语 ~ 被划线 ...