In West Calverton, most pet stores sell exotic birds, and most of those that sell exotic birds also sell tropical fish. However, any pet store there that sells tropical fish but not exotic birds does sell gerbils; and no independently owned pet stores in West Calverton sell gerbils. If the statements above are true, which one of the following must be true? ['No pet stores in West Calverton that sell tropical fish and exotic birds sell gerbils.', 'No independently owned pet store in West Calverton sells tropical fish but not exotic birds.', 'Any independently owned pet store in West Calverton that does not sell tropical fish sells exotic birds.', 'Most pet stores in West Calverton that are not independently owned do not sell exotic birds.']