A Test of Reasoning

There is a deck of cards, each of which has a number on one side of the card and a letter on the other side. This fact is a given, and you can assume it is true. Four of these cards are dealt onto a table, showing the following face up:

Conjecture: Read this CAREFULLY: "For these four cards, every card with a vowel on one side has an even number on the other."

Question: What is the smallest set of cards (list which ones) that must be turned over to test the conjecture?

Definitions: Vowel = a letter in the set {A, E, I, O, U}

Even Number is divisible by 2 without remainder,{...-4, -2, 0, 2, 4,...}

Test: An experiment that would refute (disprove) the conjecture, if the conjecture were false.

1.What is the smallest set of cards that must be turned over in order to test the conjecture? Which ones do we need to turn over?
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