【正确答案】：
SEMANTIC UNITY IS ONE OF THE CHARACTERISTICS OF IDIOMS. BEING PHRASES OR SENTENCES, IDIOMS EACH CONSIST OF MORE THAN ONE WORD, BUT EACH IS A SEMANTIC UNITY, I.E. THE MEANINGS OF VARIOUS WORDS WHICH MAKE UP THE IDIOM ARE NOT OFTEN RECOGNIZABLE IN THE MEANING OF THE WHOLE IDIOM. LIKEWISE, THE PART OF SPEECH OF EACH ELEMENT IS NO LONGER IMPORTANT. FOR INSTANCE, " TILL THE COWS COME HOME", MEANS "FOREVER" AND FUNCTIONS AS AN ADVERB. THE SEMANTIC UNITY OF IDIOMS IS ALSO REFLECTED IN THE ILLOGICAL RELATIONSHIP BETWEEN THE LITERAL MEANING OF EACH WORD AND THE MEANING OF THE IDIOM. FOR EXAMPLE, "RAIN CATS AND DOGS", WHICH MEANS RAIN HEAVILY. LITERALLY MAKES NONSENSE. 
【题目解析】：本题考查语义整体性的含义。 语义的整体性是习语的特点之一。 1.习语由短语或句子构成，每一个构成习语的词都是一个语义整体，也就是说构成习语的各个词的意义在习语的整体意义中往往是不可辨认的。同样，每个词的词性也不再重要。例如，till the cows come home直到牛回家，意思是永远，充当副词使用。 2.习语的语义整体性也反映在每个词的字面意义与习语的意义不符合逻辑中。例如，rain cats and dogs，字面意思是下猫和狗，而实际上是倾盆大雨的意思。