Which of the following is>are simple random samples?

a. As Lipitor pills are being manufactured, a quality control plan is to select every 500th pill and test it to confirm that it contains 80 mg of atorvastatin.

b. To test for a gender difference in the way that men and women make online purchases, Gallup surveys 500 randomly selected men and 500 randomly selected women.

c. A list of all 10,877 adults in Trinity County, California, is obtained; the list is numbered from 1 to 10,877; and then a computer is used to randomly generate 250 different numbers between 1 and 10,877. The sample consists of the adults corresponding to the selected numbers.