Language is a system, which consists of two sets of structures,

Language is a system, which consists of two sets of structures, or two levels, i. e.，______.
A、arbitrariness
B、productivity
C、duality
D、displacement
【正确答案】：C
【题目解析】：Language is a system, which consists of two sets of structures, or two levels, i. e.，duality.