完成句子，用所给单词的正确形式填空。
【小题1】It is required in the library regulations that any book borrowed __________ (return) by due time, or the borrower will be fined.
【小题2】Without doubt taking physical exercise regularly is an effective way to avoid ___________ (infect) with the flu.
【小题3】If you ______________ (not mention) this matter to me again yesterday, I would certainly have forgotten it.
【小题4】The famous professor __________ (refer) to in your conversation just now would come to the urgent conference.
【小题5】With a lot of difficult problems __________ (settle), the manager felt like a cat on hot bricks.