A.The Great Wall is a long wall in the world.

B.The Great Wall is the longest wall in China.

C.The Great Wall is the longest wall in the world.

【答案】C

【解析】

A长城是世界上的一堵长墙。B长城是中国最长的长城。C长城是世界上最长的长城。表达长城是世界上最长的城墙，故选C。