There are 9 cards, from 1 to 9. A. B, C, and D take four cards, and each person takes 2 cards. It is known that the sum of the two cards taken by A is 10; the sum of the two cards taken by B is 10; The difference is 1; the product of the two cards taken by C is 24; the quotient of the two cards taken by D is 3. Please tell me what card is left? ['.', '.', '.', '.']