Examine each of the following statistics. Which sampling method was most likely to have been used (simple random, systematic, stratified, cluster)?

a. A survey showed that 30 percent of U.S. businesses have fired an employee for inappropriate web surfing, such as gambling, watching porn, or shopping.

b. Surveyed doctors report that 59 percent of patients do not follow their prescribed treatment.

c. The Internal Revenue Service reports that, based on a sample of individual taxpayers, 80 percent of those who failed to pay what they owed did so through honest errors or misinterpretation of the tax code.

d. In Spain, per capita consumption of cigarettes is 1,265 compared with 1,083 in the United States.