A king has 4 prisoners about to be sentenced to death. But he decides to give them a chance.
He asks the 4 to line up in a single file. The shortest person A is at first. The tallest B is
in second. The third person C slightly shorter than B is third. And the last one D, who is slightly
shorter than C. They cannot talk to each other.

A can see noone.
B can see A.
C can see only B.
D can see B and C.

The king takes 2 black hats and 2 white hats and puts one on each. He then asks the prisoners to guess
the colour of their hat telling them that there are 2 black and 2 white hats.  HE says if one person gets
it right, they all live. If one person gets it wrong, they all die.

For 5 minutes, the prisoners are quiet. Then one of the prisoner makes a correct guess and they are all
set free. So who gets it correct?. And how?.