Complete the following statements with proper words or expressions according to the course book.The idiom “by twos and threes” cannot be turned into “by threes and twos”. It shows that unlike free phrases, the structure of an idiom is to a large extent( )．
【正确答案】：UNCHANGEABLE
【题目解析】：习语“by twos and threes（三三两两）”不能变成“by threes and twos”。这表明，与自由短语不同，习语的结构在很大程度上是不可改变的。
