You are shown a set of four cards placed on a table, each of which has a number on one side and a symbol on the other side. The visible faces of the cards show the numbers 2 and 7, and the symbols , and O. Which card(s) must you turn over in order to test the truth of the proposition that "If a card has an odd number on one side, then it has the symbol on the other side"? Explain your reasoning by deciding for each card whether it should be turned over and why