6. John wanted to determine the average cost of renting a two-bedroom apartment in the Indianapolis area. To save time, he called 20 college friends who lived in two-bedroom apartments and asked them how much rent they paid. He then used these figures to get his estimate. Answer the question below based on this story.

Which of the following would John have to do to remove bias from his study?

Group of answer choices

a. Increase his sample size.

b. Randomly select 20 of his friends rather than just picking them.

c. Obtain a list of all rentals in Indianapolis and randomly select 20.

d. Increase his population size.