It's sometimes difficult to either group yourself with other riders OR in this case seperate yourself. Whoever defined the A, B, and C classes probably didnt have the strictest of rules, yet people tried to adhere to them as closely as possible.
Lets say you spend a summer in the C group, averaging about 15-16 mph over a 20 mile course. You feel yourself get a little better as time goes on, so you decide to step up with the boys and hit the B group. According to most resources I found, the B group will keep a 17-19 mph pace. Ok so your ready. But wait...are you?
You'll find that in some B groups, the riders there are reaching just like you did - and they want to become an A. This sucks for the C+ B- rider, since he or she will get truly hammered on the ride that is averaging 22 mph over a 30 mile course. Its no wonder B's are dropping like flies. (no pun intended)
Perhaps its time that A's seperate themselves from the B's and create a group where they can speed their way through the course without effecting the B ride. I, for one, could do without the cramps.