Brain Teaser

Here is a small brain teaser (don't worry no probability involved).

You are shown four cards.  Each card has a letter on one side, and a number on the other side.  You see only one side of each card; you see “A B 2 1” on the visible faces of the four cards.

 Your task is to check to see if the following rule is obeyed by the cards. 

 The rule: if a card has a vowel on one side, then it has an even number on the other side.

Which of these four cards would you need to turn over to check that the rule is being obeyed?  You may choose more than one card if you wish.