Explain whether each of the following constitutes data collected from a population or a sample.
a. Opinions on a certain issue obtained from all adults living in a city.
b. The price of a gallon of regular unleaded gasoline on a given day at each of 28 gas stations in the Miami, Florida, metropolitan area.
c. Credit card debts of 100 families selected from a given city.
d. The percentage of all U.S. registered voters in each state who voted in the 2012 Presidential election.
e. The number of left-handed students in each of 50 classes selected from a given university.