When 10 000 Molecules of ATP Are Hydrolyzed to ADP

When 10 000 molecules of ATP are hydrolyzed to ADP and _i in a test tube, about half as much heat is liberated as when a cell hydrolyzes the same amount of ATP. Which of the following is the best explanation for this observation?

A) The hydrolysis of ATP in a cell produces different chemical products than does the reaction in a test tube.
B) Reactant and product concentrations in the test tube are different from those in the cell.
C) The reaction in cells must be catalyzed by enzymes, but the reaction in a test tube does not need enzymes.
D) Cells are less efficient at heat production than nonliving systems.
E) Cells are open systems, but a test tube is an isolated system.