答案：
1. She had a guilty conscience (感到内疚) about not telling the police what had actually happened. (本题考查形容词搭配，have a guilty conscience about意思为“内疚”)
2. Many a time has the child been told not to play with fire (警告过那孩子不要玩火) but he turns a deaf ear to the warnings.
(本题考查倒装结构，此句中“but”是并列连词，连接两个并列句，状语many a time放在句首，句子要倒装)
3. In manufacturing
industries some types of plastics have become a substitute for metal (取代了金属).
(本题考查名词辨析与搭配，名词substitute常考试大用于“substitute for”结构中，意为“替代”)
4.Without a sure supply of water, farming in that
area remains at the mercy of the weather (靠天吃饭) (本题考查mercy的用法，at the mercy of 意为“任凭…的摆布”“完全收…的支配”)
5.It was because she was inexperienced that she didn’t know how to deal with the situation (她不知道如何应对这种形式)
(本题考查强调句型，这是一个it is …that…的强调句型，强调部分
because she was
inexperienced)
