The long run is best defined as a time period () that is longer than two years. () during which at least one input cannot be changed. () that is long enough to change all factors of production. () during which consumer demand for goods and services changes. One thing that distinguishes the short run from the long run is the number of months considered. O () the presence of marginal costs. () whether any costs are fixed. () explicit costs. Which is most likely to be a natural monopoly? () a firm that owns all of the world's gold and is therefore the only firm that sells gold jewelry () a firm that provides electricity to all of the homes in Los Angeles () afirm selling health food that has no local competitors () a firm that has a patented low calorie pizza recipe and is therefore the only firm that that can sell diet pizza Firms in monopolistic competition () participate in markets where barriers to entry are present. () differentiate their products. () are price takers