【正确答案】：
THE DIFFERENCES BETWEEN SYNONYMS BOIL DOWN TO THREE AREAS: DENOTATION, CONNOTATION AND APPLICATION.
1)DIFFERENCE IN DENOTATION. SYNONYMS MAY DIFFER IN THE RANGE AND INTENSITY OF MEANING. SOME WORDS HAVE A WIDER RANGE OF MEANING THAN OTHERS. FOR GROUP (A). THE VERB UNDERSTAND IS USED IN A MUCH MORE EXTENDED SENSE THAN COMPREHEND. WHATEVER IS COMPREHENDED IS UNDERSTOOD, BUT IN MANY CASES, COMPREHEND CANNOT TAKE THE PLACE OF UNDERSTAND.
2) DIFFERENCE IN CONNOTATION. BY CONNOTATION, WE MEAN THE STYLISTIC AND EMOTIVE COLOURING OF WORDS. SOME WORDS SHARE THE SAME DENOTATION BUT DIFFER IN THEIR STYLISTIC APPROPRIATENESS. FOR GROUP (B), ANSWER IS NATIVE AND NOT STYLE-SPECIFIC WHEREAS RESPOND IS A BORROWED WORD AND IS MORE FORMAL. THE BORROWINGS ARE MORE APPROPRIATE FOR FORMAL AND TECHNICAL WRITING.
3)DIFFERENCE IN APPLICATION. MANY WORDS ARE SYNONYMOUS IN MEANING BUT DIFFERENT IN USAGE IN SIMPLE TERMS. THEY FORM DIFFERENT COLLOCATION AND FIT INTO DIFFERENT SENTENCE PATTERNS. FOR GROUP(C), ALLOW AND LET ARE SYNONYMS BUT WE ALLOW SB TO DO STH AND LET SB DO STH.
【题目解析】：同义词的区别可以归结为三个方面:外延、内涵和应用。 1)外延的区别：同义词的强弱程度上可能不同，有些词比其他词具有更广的意义。在（a）组中，动词“understand（理解）”的用法比“comprehend（领会）”的意义更为广泛。凡是能领会的都能理解。但在许多情况下，comprehend不能代替understand。2)内涵的差异：我们所说的内涵是指文字的语体和情感色彩。有些单词具有相同的含义，但在文体恰当性上有所不同。对于（b）组来说，answer（回答）是本地的，而不是特定于风格的，而respond（响应）是借来的单词，并且更正式。借词更适合于正式和技术性写作。3)应用上的差异：许多词在意义上是同义的，但用法并不同。它们形成不同的搭配，适合不同的句型。对于（c）组，allow和let是同义词，但是常见的搭配是allow sb. to do sth和 let sb. do sth。