Which of the following correctly describes a compound? (4 points) a The atoms are chemically bonded together, and they retain their individual physical and chemical properties. b The atoms are not chemically bonded, and there is no set ratio for how the atoms can combine together. c The atoms can only combine in fixed ratios, and they can only be separated by a chemical change. d The atoms do not retain their individual chemical properties, and they can be separated by physical means.