There are far fewer independent bookstores than there were 20 years ago, largely because chain bookstores prospered and multiplied during that time. Thus, chain bookstores' success has been to the detriment of book consumers, for the shortage of independent bookstores has prevented the variety of readily available books from growing as much as it otherwise would have. Which one of the following is an assumption on which the argument relies? ['Independent bookstores typically do not sell the kinds of books that are available in chain bookstores.', 'Book consumers would be better off if there were a greater variety of readily available books than there currently is.', 'The average bookstore today is larger than the average bookstore of 20 years ago.', 'Some book consumers value low prices more highly than wide selection.']