Explain how these statements differ

B and C are correct as written, but B requires some clarification. A is incorrect because enzyme molecules are not consumed in chemical reactions and will continue to perform until something external to the reaction causes it to stop. B is correct if the product is removed from the system, but if the product remains, some of it will break down into substrates; therefore they would not be consumed. In D, if product molecules are removed, the reaction would continue to proceed to form new product (assuming enzymes and substrates are still available). Chemical reactions continue if conditions are favorable, including presence of substrate and enzyme and fairly constant temperature and pH. In living systems, most chemical reactions do not have a steady supply of substrate; for example, the chemical reactions of digestion will stop when the small intestine is empty.