In each of the following situations, explain whether an externality is present.

a. Mine safety has improved in recent years. Nonetheless, mining accidents result in 50 to 100 deaths per year and thousands of lost workdays due to injury.

b. Large brokerage and financial service companies conduct intensive introductory training programs for new hires, many of whom, once trained, leave the company within the first year to work for competitors.

c. The volume of e-mail spam has grown exponentially in the last 5 years.

d. A husband and wife who have put off buying a house suddenly find themselves priced out of the market by rocketing real-estate prices.