A set of 4 cards are placed on a table. Each card has a person's age on one side and the name of a drink on the other side. The visible card faces are showing 50, 17, beer, and Pepsi, respectively. What cards should you turn over to test the truth of the proposition that if a card is showing an alcoholic drink on one side, then the age shown on the other side of the card is at least 21?