Have you ever dreamed of spending a night in a Ferris wheel? (摩天轮)Your dream may come true soon. French architects(建筑师) are planning to build a water wheel(水车) hotel on the bank of the Seine.

    The water wheel will look like the London eye. The London Eye lies in the Thames which is the world first and the third biggest wheel. The water wheel hotel's pods (吊舱) will become hotel rooms. The water wheel will be made of wood and it will have 19 pods. There will be a bathroom in each pod.

    Although sleeping in such a hotel will be very unusual, the experience isn't just for rich people. A night will only cost about 300 euros(欧元).

一个房间有3盏灯，那么9个房间有多少盏灯？

one的序数词是____.