*riddle*

There was this wizard who caught 3 men {let's just say Mr.A, B and C}. The wizard wanted to kill all three of them, but they pleaded and so the wizard gave them a chance.
The wizard put the 3 men on flight of stairs, A first {A is on top of the flight of stairs} followed by B {B is in the middle} , and then C {C is at the base} , in this order:
------------------------A
---------------------------
----------------------------B
------------------------------
-------------------------------C
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They were stationed in such a way that A can see B and C, and B can only see C, but can't see A, and C can see none.

They were then blindfolded, and on each of their head, he put a coloured cap, the colour of the cap can only either be black or white, however, there will be either 2 black 1 white, or 2 white and 1 black, either way the 3 men don't know, all that they know is that there will be either black or white cap on their head, and that there will be either 2 white or 2 black cap on the 3 of them, which one they don't know.

So the wizard want them to tell him what coloured hat is on their head within the next 5 seconds after he removed the blindfold. Any one who could answer correctly would save the life of all 3 men.

Then, the wizard removed the blindfold......

After 4 seconds, the one in the middle {B} answer the question, and it was correct, how did he do it?

Hint: Whatever the answer is doesn't matter, even if it is white or black, the most important thing is how he got the answer.

Hint: What did the man on top see {A}??