A bank analyst wants to ask customers who have a mortgage with the bank about the customers' home-buying experiences. Which of the following best describes a random sample of customers?
a) The analyst uses a computer program to draw 78 customers at random and selects these customers.
b) Every set of 78 customers is equally likely to be drawn by the computer program.
c) The analyst forms 6 groups of customers based on the customers' mortgage amounts. Then, she selects 13 customers at random from each group.
d) The analyst spends a day calling mortgage customers and selects the first 78 customers who answer because these customers are easily accessible.