The teacher had the students __________ all the words they didn’t know.

问题描述:

The teacher had the students __________ all the words they didn’t know.
A.looked up B.looking in C.look up D.look at

have sb do sth是固定句型,表示让某人去做某事.
look up是固定短语,表示在字典中查阅(单词)
所以选C