In the last example (with Jon), there's a 'must' in there too. Is 'must' not an indicator for a necessary condition? How we do know to follow the plan for the 'or' instead of the 'must'?

I'm still unclear why A is more wrong than C. It seems like they both require some stretches of inference and the explanation didn't help me understand why A was the "wrong" kind and C was the acceptable (albeit not great) kind. Clarification?