Unlike free phrases, the structure of an idiom is to a large extent ( ) , for instance, by twos and threes cannot be turned into by threes and twos.
【正确答案】：UNCHANGEABLE
【题目解析】：本题考查习语结构的稳定性。与自由短语不同，习语的结构在很大程度上是不可改变的。习语具有稳定性包括，习语的语序不能倒装或改变。例如by twos and threes不能被转变为by threes and twos。unchangeable：不可改变的
