There is a tour group in Shanghai to travel to Sanya, Hainan. The most economical mode of transportation is () A. Railway B. Aviation C. Sea D. River.

Understand the characteristics of various modes of transportation: road transportation is flexible, door-to-door and suitable for short-distance transportation; Air transportation is fast, but small in size, suitable for long-distance transportation that is urgently needed; Railway transportation is fast and large, which is suitable for long-distance transportation with large volume; The water transport volume is large, but the speed is slow, which is limited by the region and suitable for long-distance transportation with large volume between two seaports; Pipeline transportation is underground and is generally suitable for long-distance transportation of oil, natural gas and water. Shanghai to Sanya, Hainan, both cities belong to seaports, so shipping is the most economical choice.

So choose: C.