Someone took 8 prints, 4 red and 4 green, and gently glued them to the foreheads of three people A, B, and C, two prints each. In this way, each of them can see the print on the other person's forehead, but not the two on their own forehead and the two in the host's pocket. The moderator asked everyone in order if they knew what color the print on their head was:
A: I don't know
B: I don't know
C: I don't know
A: I don't know
B: Yes
So what color are the two prints on B's forehead? How did he know?