For each scenario, assess whether an externality is present.

a. Vaccinating children against influenza reduces its incidence among the elderly.

b. Newly graduated nurses flock to teaching hospitals for training. After working for a year, many leave to work for competitors.

c. A couple who planned to move to Florida to retire find that the plummeting housing market has wiped out their equity.

d. Physicians complain that they spend a third of their time explaining to patients why television advertisements about medications for their conditions do not apply to them.