So, A, B is the same thing as B, A. A, C is the same thing as C, A. A, D is the same thing as D, A. B, C is the same thing as C, B. B, D is the same thing as D, B. C, D is the same thing as D, C. And so we'd be left with, if we correct for the double counting, we're left with one, two, three, four, five, six combinations, six possible ways of choosing two from a pool of four, especially when you don't care about the order in which you choose them. Thank you.