The Food and Drug Administration used key hearings today to define the battle over regulating cigarettes down to one simple proposition. "The bottom line question: Is nicotine in cigarettes addictive?" FDA commissioner David Kessler asked a parade of experts at the agency's Washington hearings. Most of the guests said yes. "Then why the debate?" Kessler asked. The drug experts, who called the fight with tobacco companies over addiction "semantic," posed some intriguing possibilities for regulation. One proposal: limit the nicotine in individual cigarettes to make smoking "really a free choice." That way, most people could smoke up to a pack a day without becoming addicted.