词汇指导：四级语法中表示转折或对比

        1) but表示转折，while表示对比。
        Some people love cats, while others hate them.
        
        典型例题　
        --- Would you like to come to dinner tonight?
        --- I'd like to, ___ I'm too busy.
        A. and B. so C. as D. but
        答案D。but与前面形成转折，符合语意。而表并列的and， 结果的so，原因的as都不符合句意。
        
        
        2) not…but… 意思为"不是……而是……"
         not 和but 后面的用词要遵循一致原则。
        They were not the bones of an animal, but (the bones) of a human being.
        
        比较and和or
        
        1) 并列结构中，or通常用于否定句，and用于肯定句。
        
        2) 但有时and 也可用于否定句。请注意其不同特点：
        There is no air or water in the moon.
        There is no air and no water on the moon.
        在否定中并列结构用or 连接，但含有两个否定词的句子实际被看作是肯定结构，因此要用and。
        
        典型例题
        ---I don't like chicken ___ fish.　
        ---I don't like chicken, ___ I like fish very much.
        A. and； and B. and； but　C. or； but　D. or；and
        答案C。否定句中表并列用or, but 表转折。
        
        判断改错：
        (错) We will die without air and water.
        (错) We can't live without air or water.
        (对) We will die without air or water.
        (对) We can't live without air and water.
        

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