Can you solve the hat riddle? (opens in new tab)

You and 3 amigos are buried neck deep in sand in a prison yard\. The guard says if one of you can guess your own hat colour you can all go free\. You know there are two black hats and two white hats and you can only see the hats of the people in front of you\. A minute passes in silence and then one of you shouts out a correct answer\. Who shouted? A, B, C or D?