First, I think weak B league (and OK A teams) would be the level of HS varsity. Play A if you were the star of your varsity. Some C teams, as McGizzle said, are actually playing at the B level, but they play in C so they beat up on the weaker teams.
I'm just going to say a handful of players in C can dunk (though you can't dunk in a game, or you receive a T). At the B and C level, it's just about if you're physical, play with good team chemistry and players have good fundamentals.
There are teams with only one or even not any athletic players, and can still win games.
Really make sure your team has size, and people who will box out. Guys in Sunset are lazy and don't crash the board. But even good teams in C will play physical and punish you if you don't box out.
Also, make sure your team is at least 7 deep, or a committed team. My team had to play 5 vs. 4 once and we lost by 40.