I would appreciate if you can solve this problem.

Each of the cards shown below has a number on one side and a letter on the other. How many of the cards must be turned over to prove the correctness of this statement for these cards: "If a card has a vowel on one side, then it has a prime number on the other side" ?

[ A ] [ B ] [ C ] [ 3 ] [ 4 ] [ 5 ]