Spoiler C calls out that he is wearing a black hat. Why is he 100% certain of the color of his hat? After a while, C comes to the realization that he must answer. This is because D can't answer, and neither can A or B. D can see C and B, but can't determine his own hat color. B can't see anyone and also can't determine his own hat color. A is in the same situation as B, where he can't see anyone and can't determine his own hat color. Since A, B, and D are silent, that leaves C. C knows he is wearing a black hat because if D saw that both B and C were wearing white hats, then he would have answered. But since D is silent, C knows that he must be wearing a black hat as he can see that B is wearing a white hat.
You are stuck in a house made of four walls, each with a window. All four walls point south. A bear walks past on of the windows. What color is the bear? Spoiler White. You're on the the north pole; it's a polar bear.