For each scenario, assess whether an externality is present.
a. Vaccinating children against influenza reduces its incidence among elderly.
b. Newly graduated nurses flock to reaching hospitals for training. After working for a year, many leave work for competitors.
c. A couple who planned to move to Florida to retire find that the plummeting house market has wiped out equity.
d. Physicians complain that they spend a third of their time explaining to patients why television advertisement about medications for their conditions do not apply to them.