Passage Summary: You can't say something isn't addictive just because some people can quit; a proper definition of addiction must require that most people find it very hard to stop.

Reasoning: A substance is only addictive if withdrawal causes extreme difficulty for most users, and the ability of some users to quit is not enough to prove a substance is nonaddictive.

Analysis: The expert establishes a necessary condition for addictiveness: if a substance is addictive, then withdrawal must be extremely difficult for most users. Using formal logic, we can also infer the contrapositive: if withdrawal is not extremely difficult for most users, then the substance is not addictive. The expert also explicitly rejects the idea that 'some people can quit' is a sufficient condition for 'nonaddictiveness.' Look for an answer that stays strictly within these logical boundaries without making outside assumptions.