Which of the following statements about the competitive inhibition of an enzyme -catalyzed reaction is correct?
选项：

A:The Vmax for a reaction remains unchanged in the presence of a competitive inhibitor.;
B:The Km for a reaction remains unchanged in the presence of a competitive inhibitor.;
C:The Vmax and Km (Michaelis coastant) for a reaction are unchanged in the presence of a competitive inhibitor.;
D:A competitive inhibitor and substrate can bind simultaneously to the enzyme.