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You need to choose 3 cards, in which each of the cards' features (shape, color, number, shading), looked at one-by-one, are the same on each card, or are different on each card. All of the features must separately satisfy this rule. In other words: shape must be either the same on all 3 cards, or different on each of the 3 cards; color must be either the same on all 3 cards, or different on each of the 3, etc.