The U.S. Post Office finds that it now has extra costs associated with decontaminating first class mail for novel coronavirus. It is considering a rate hike, but it will only be successful in raising more revenue to pay for these additional costs if ( )
选项：

A:there are many substitutes for first class mail service.;
B:no anthrax is found on the mail.;
C:the demand for first class mail service is inelastic.;
D:the rate increase is a very large one.