【正确答案】：
（1）HISTORICAL REASON. A WORD IS REMAINED FOR A NAME THOUGH THE MEANING HAS CHANGED BECAUSE THE REFERENT HAS CHANGED. TAKE "PEN" FOR EXAMPLE. ORIGINALLY, IT DENOTED "FEATHER". NOW, THE NAME IS STILL KEPT. ATOM USED TO MEAN, IN GREEK, "ANY OF THE INDIVISIBLE PARTICLE". NOW ATOM IS NOT THE SMALLEST AND CAN BE DIVIDED INTO EVEN SMALLER PARTICLES, HENCE THE ABANDONMENT OF THE ORIGINAL MEANING. 
（2）CLASS REASON. THE ATTITUDES OF CLASSES HAVE ALSO MADE INROADS INTO LEXICAL MEANING IN THE CASE OF ELEVATION OR DEGRADATION. FOR INSTANCE, "CHURL, HUSSY, WENCH, VILLAIN" AS WE ALREADY KNOW WERE ORIGINALLY NEUTRAL IN COLOUR BUT HAVE ALL DOWN-GRADED AS "ILL-MANNERED OR BAD PEOPLE".
（3）PSYCHOLOGICAL REASON. THE ASSOCIATED TRANSFER OF MEANING AND EUPHEMISTIC USE OF WORDS, ETC., ARE OFTEN DUE TO PSYCHOLOGICAL FACTORS. TAKE "COPPERHEAD" FOR EXAMPLE. DURING THE AMERICAN CIVIL WAR IT WAS EMPLOYED TO REFER TO THOSE NORTHERNERS WHO WERE SECRETLY AIDING AND ABETTING THE SOUTH. 
【题目解析】：（1）历史原因。一个词仍然是一个名字，尽管意思已经改变，因为所指物已经改变。以“笔”为例。最初，它表示“羽毛”。现在，这个名字仍然保留着。原子过去在希腊语中的意思是“任何不可分割的粒子”。现在原子不是最小的，可以分成更小的粒子，因此放弃了原来的意义。（2）阶级原因。阶级的态度也在升格或降格的情况下进入了词汇意义。例如，“丘尔、侯赛因、丫头、恶棍”，这些我们已经知道的词，最初是中性的，但都被降级为“不礼貌的或坏的人”。（3）心理原因。词义的联想转移和委婉使用等，往往是由于心理因素。以“铜斑蛇”为例。在美国内战期间，它被用来指那些秘密帮助和教唆南方的北方人。