Hi, we are looking to justify the idea that they should vote yes. That means that our right answer must tell us that if certain conditions are met, then you should vote yes. In other words, vote yes should be our necessary condition.
In B, it is a sufficient condition. B tells us we should vote no unless it will benefit us, or Not B then vote no Vote yes then benefit
Neither of these enable us to say we should vote yes, and vote yes is only a sufficient condition.
