How about:
A = I'm quick enough to race at the front of U17/U19 or anywhere in Expert/Elite/U23
B is 80% "I'm quick enough to race in A but I'm a goddanm sandbagger" and "I race A grade with my local club, and at crits, and everywhere else, but Elite level rider X is in A so I'll go B" and "I haven't been riding for x weeks" and "I have a cold" and "I cbf racing today" and 20% "I don't fit the criteria for C"
C = This is my first year racing, xor I didn't finish a race last year in the top 5 anywhere in the state.
Some additional exclusion criteria.
1. If you can define the terms VO2max, FTP or Negative Split, you're probably not C grade.
2. If you've ever actually done a test for any of the above, it's very very likely you're not C grade.
3. If you've ever podium-ed in C grade in a reasonable sized field in a previous years season, you're not C grade anymore.
4. If you've ever raced A grade (ever), you're not C grade. Ever again.
4a. No, I don't care that you haven't ridden for 12 months.
5. If you're a B grade roadie, you're not C grade.
6. If you're an A grade roadie, you can probably handle a bike well enough even if "you're not a mountain biker". You're not B grade.
7. If you can race a 100km marathon, you're probably not C grade.
8. If you're having problems because you've caught traffic of the tail end of the grade above on the first lap, you're in the wrong grade.
Okay there, I've had my fun for the night