Economies of scale are a barrier to entry because: a. New entrants face high unit costs either because they enter at sub-optimal scale, or they make a large-scale entry that initially operates with substantial excess capacity. b. New entrants are positioned at the top of their learning curve. c. New entrants face a risk of retaliation from the incumbents whose large scale of operation allows them to flood the market. d. New entrants are uncertain about their future costs which discourages them from making investments.