18. Which one of the following statements is true of enzyme catalysts? (A) Their catalytic activity is independent of pH. (B) They are generally equally active on D and L isomers of a given substrate. (C)They can increase the equilibrium constant for a given reaction by a thousand fold or more. (D) They can increase the reaction rate for a given reaction by a thousand fold or more. (E) They must be at the same concentration as their substrate to be effective.