a. Because those what exit jokes are getting really old.

b.  Variety is the spice of life.

c. Federal law.

The correct answer is c.

As a condition of receiving federal funds, states must follow federal highway guidelines.  The 2009 Manual of Uniform Traffic Control Devices (MUTCD) “changes guidelines for how exits are to be numbered. The new standard says that if any of the exits in a series is going to use a suffix, all of the exits in the series must use a suffix.”  Voila!