An idiom differs from a free phrase in that the former is（ ）and

An idiom differs from a free phrase in that the former is（ ）and the latter is not.
A、structurally changeable
B、​semantically analyzable
C、structurally fixed
D、easily understood
【正确答案】：C
【题目解析】：成语与自由短语的不同之处在于前者是结构固定的，而后者则不是。