Structural _____ is reflected in such idioms as by twos and thre

Structural _____ is reflected in such idioms as by twos and threes, tit fortat,whose word order cannot be changed.
【正确答案】：stability
【题目解析】：由所给信息看，这句话强调的是习语结构的稳定性，因为这些习语不可以更改或者替换。