2. If a ball is rolling, it should roll forever unless acted upon by an outside force. Of course, a rolling ball never seems to be in danger of rolling forever. That force is called .

How is the central idea developed further? (a) the author elaborates on the treatments for allergies in adults and how they work. (b) the author quotes the reactions of experts and the methods used to conduct the study. (c) the author compares the different rates of food allergies shown in children and adults. (d) the author indicates the limitations of the study because of differences in health care.