18. A) To protect her from getting scratches.

　　B) To help relieve her of the pain.

　　C) To prevent mosquito bites.

　　D) To avoid getting sunburnt.

　　W: These mosquito bites are killing me. I cant help scratching.

　　M: Next time you go camping, take some precaution, say wearing long sleeves.

　　Q: Why does the man suggest the woman wear long sleeves?

　　C)。四个选项都是不定式作目的状语，听音时要注意说话人做事的目的。对话中女士埋怨蚊子咬得她忍不住直挠，男士建议她下次野营时要采取防护措施――穿长袖衣服。可见男士建议女士穿长袖衣服是为了预防蚊子叮咬。故本题答案为C)。