判断以下三组词语是否同义词。
a) Happy, Joyful
b) Big, Enormous, Small
c) Car, Automobile, Train
a) Happy and Joyful are similar in meaning, but not exact synonyms.
b) Big, Enormous, and Small are antonyms, not synonyms.
c) Car, Automobile, and Train are not synonyms, but could be considered hyponyms under the broader category of transportation.