State which sampling method was used.

a. Out of 14 banks in a city, a researcher selects one bank and records the number of savings deposits made in a one-day period.

b. A researcher divides a group of students who are majoring in criminal justice as male or female, and then divides the group further as first-years, sophomores, juniors, or seniors. Then 8 students from each group are given a survey to answer questions about the program.

c. A researcher numbers the subscribers to a movie rental business, selects 1000 subscribers using random numbers, and records the number of movies rented by each for the month of January.

d. On an assembly line, every 20th automobile is selected and checked for paint defects on the car. The purpose is to ensure that the painting apparatus is working properly.