In a yearlong study, half of the participants were given a simple kit to use at home for measuring the cholesterol level of their blood. They reduced their cholesterol levels on average 15 percent more than did participants without the kit. Participants were selected at random from among people with dangerously high cholesterol levels. Which one of the following, if true, most helps to explain the study's finding? ['The lower a blood-cholesterol level is, the less accurate are measurements made by the kit.', 'Participants using the kit reported that each reading reinforced their efforts to reduce their cholesterol levels.', 'All the participants in the study showed some lowering of cholesterol levels, the most striking decreases having been achieved in the first three months.', 'Participants with the kit were more likely to avoid foods that lower cholesterol level.']