The diagram to the right depicts the market for cough medicine in a small town. The quantity of cough medicine is measured in bottles sold per?week; price is measured in dollars. The townspeople are happier and healthier because people who have colds and flus buy and take the medicine and reduce the spread of infection. Note that demand curve D1 represents the marginal private benefit of cough medicine. What is the benefit of the externality per bottle of cough?medicine?

The benefit of the externality is ?$( )per bottle of cough medicine.