请问C选项为什么不对？

问题如下：

Four countries operate within a customs union. One country proposes moving to a common market structure. What additional level of economic integration between the countries would most likely arise if this change took place? They would:

选项：

A.establish common trade barriers against non-members. B.begin to allow free movement of the factors of production. C.establish common economic institutions and coordination of economic policies

解释：

A common market structure incorporates all aspects of the customs union and extends it by allowing free movement of factors of production among members.

中文解析：

共同市场结构包括关税同盟的所有方面，并通过允许生产要素在成员国之间自由流动来扩大关税同盟。