mismanaged postal service

The postal service is badly mismanaged. Forty years ago, first-class letter delivery cost only three cents. Since then, the price has increased nearly tenfold, with an actual decrease in the speed and reliability of service.

Each of the following statements, if true, would tend to weaken the argument above EXCEPT:

(A) The volume of mail handled by the postal service has increased dramatically over the last forty years.

(B) Unprecedented increases in the cost of fuel for trucks and planes have put severe upward pressures on postal delivery costs.

(C) Private delivery services usually charge more than does the postal service for comparable delivery charges.

(D) The average delivery time for a first-class letter four decades ago was actually slightly longer than it is today.（E）

(E) The average level of consumer prices overall has increased more than 300 percent over the last forty years.

The OA is E. Can someone explain why C is not the correct answer? In my opinion, C doesn't strengthen nor weaken the argument. It is completely unrelated to the argment.