【正确答案】：
COMPOUNDS DIFFER FROM FREE PHRASES IN THREE ASPECTS.
1)PHONETIC FEATURES. IN COMPOUNDS THE WORD STRESS USUALLY OCCURS ON THE FIRST ELEMENT WHEREAS IN NOUN PHRASES THE SECOND ELEMENT IS GENERALLY STRESSED IF THERE IS ONLY ONE STRESS. AS IN SILKWORM, THE STRESS IS ON SILK. 
2)SEMANTIC FEATURES. COMPOUNDS ARE DIFFERENT FROM FREE PHRASES IN SEMANTIC UNITY. EVERY COMPOUND SHOULD EXPRESS A SINGLE IDEA JUST AS ONE WORD. FOR INSTANCE, A GREEN HAND IS AN "INEXPERIENCED PERSON", NOT A HAND THAT IS GREEN IN COLOR.
3)GRAMMATICAL FEATURES. A COMPOUND TENDS TO PLAY A SINGLE GRAMMATICAL ROLE IN A SENTENCE, FOR EXAMPLE, A VERB, A NOUN, OR AN ADJECTIVE. BAD-MOUTH USED AS A VERB CAN TAKE THE THIRD PERSON SINGULAR -S AND THE PAST TENSE MARKER -ED.
【题目解析】：本题考查复合词不同于自由短语的特点。1、语音特点。在复合词中，重音通常出现在第一个构词成分上，而在名词短语中，重音一般在第二个词上（如果这个词只有一个重音）。比如复合词silkworm，重音在silk上。2、语义特点。复合词与自由短语在语义上的统一性不同。每一个复合词都应该表达一个单一的意思。例如，a green hand是指“新手”，而不是“一只绿色的手”。3、语法特点。复合词在语法上独立作成分，如作动词、名词或形容词等。比如：bad-mouth用作动词时，可以使用第三人称单数和过去式标记。