【正确答案】：
(1) HISTORICAL REASON. INCREASED SCIENTIFIC KNOWLEDGE AND DISCOVERY, OBJECTS, INSTITUTIONS, IDEAS, SCIENTIFIC CONCEPTS CHANGE IN THE COURSE OF TIME. (2)CLASS REASON. DIFFERENT CLAUSES HAVE DIFFERENT LANGUAGES AND ATTITUDES. (3)PSYCHOLOGICAL REASON. PEOPLE CHANGE WORD-MEANING OWING TO VARIOUS PSYCHOLOGICAL MOTIVES, 
【题目解析】：(1) Historical reason. Increased scientific knowledge and discovery, objects, institutions, ideas, scientific concepts change in the course of time. (2)Class reason. Different clauses have different languages and attitudes. (3)Psychological reason. People change word-meaning owing to various psychological motives,