Vons, a large supermarket in Grover Beach, California, is considering extending its store hours from 7:00 am to midnight, seven days a week, to 6:00 am to midnight. Discuss the sampling bias in the following sampling strategies:
a. Mail a prepaid envelope to randomly selected residents in the Grover Beach area, asking for their preference for the store hours.
b. Ask the customers who frequent the store in the morning if they would prefer an earlier opening time.
c. Place an ad in the local newspaper, requesting people to submit their preference for store hours on the store's website.