9. (10 points) There are 20 rooms, some with lights on and some with lights off. The people in these rooms all hope to be in the majority. Now, starting from the person in the first room, if more of the remaining 19 rooms have their lights on, they will turn the light on, otherwise they will turn the light off. If initially, 10 rooms have their lights on and 10 rooms have their lights off, and the light in the first room is on. Then, after all 20 rooms have gone through this process, the number of rooms with the lights off is ( ).
A. 0
B. 10
C. 11
D. 20
【Answer】Solution: Since at the beginning, 10 rooms have their lights on and 10 rooms have their lights off, and because the light in the first room is on, the person in the first room will see 9 rooms with lights on and 10 rooms with lights off, and then he will turn off the light,

After that, no matter whether someone from a room with the light on or off comes out to check, there will always be more lights off, i.e.: at the end of the round, all the lights will be off, hence the answer is: D.