A student designs an experiment to test substances X, Y, and Z, to determine which one is a catalyst for the reaction: A + B ® C. Only one of the unknown substances is a catalyst, and the others are nonreactive with A, B, or C. When 10 mL of A is added to 10 mL of B, the reaction takes twenty seconds. Bubbles form when the product C is created. The student prepares three test tubes, each containing both reactants A and B. She adds unknowns X, Y, and Z to test tubes 1, 2, and 3, respectively. She then times the reaction in each test tube from the point when the unknown is added until bubbling stops. How can the student identify the catalyst?