A company wants to determine which of its snack options the customers' favorite is. Which two sampling methods produce unbiased representative samples of the population?
a The company mails out surveys to all of its customers and uses the data from the surveys that are returned.
b The company assigns each customer a number using a random number generator and selects the first 20% of the numbers to survey.
c The company surveys every employee at the company.
d The company surveys every 5th customer on a randomized list of all customers.
e The company runs a commercial offering free snacks to whoever calls in and answers the survey.