Because quitting smoking is very stressful and leads to weight gain, it is difficult to do. The key to quitting, however, may be as simple as replacing an unhealthy activity with a healthy one. In one study, half of those attempting to quit were assigned to a smoking-cessation program alone, and the other half were assigned to the same program plus fifteen weeks of aerobic exercise. At the one-month mark, none in the first group had quit, but 40 percent of those in the second group had not smoked. Each of the following, if true, provides some support for the argument EXCEPT: ['Regular exercise prevents weight gain.', 'Each group in the study included four hundred randomly selected participants.', 'Nonsmokers accustomed to regular exercise do not gain weight when they stop exercising.', "Aerobic exercise can stimulate the brain's production of endorphins, which reduce tension."]