How do these two substances compare? explain. a. both substances are pure substances and can be classified as molecules. o2 is considered a compound, because it made up of two different elements, whereas h2o is not a compound. b. both substances are pure substances and can be classified as molecules. h2o is considered a compound, because it made up of two different elements, whereas o2 is not a compound. c. both substances are pure substances and can be classified as molecules. each one is also considered a compound, because they have elements bonded together. d. both substances are pure substances and can be classified as molecules. neither is called a compound because they have only one element in each molecule.