What are the major sources of English synonyms?

What are the major sources of English synonyms?
【正确答案】：Sources of synonyms are；borrowing， dialects and regional English, figurative and euphemistic use of words, and coincidence with idiomatic expressions.
【题目解析】：同义词在英语词汇中占有很大的比重，它的形成原因是更好理解同义词的重要手段。