1.C 前缀　(in grammar) an affix added to the beginning of a word
2.C 人名前的称谓　a title used before a person's name

In the word "unimportant"，"un-”is a prefix.在单词unimportant中，un是前缀。
　He was always addressed by his surname with prefix of Mr.　人们总是在他的姓名前加上先生来称呼他。