In​ 2008, cruise ship lines announced they were increasing prices from​ $7 to​ $9 per person per day because of increased fuel costs. According to one​ analyst, fuel costs for Carnival​ Corporation's 84-ship fleet jumped​ $900 million to​ $2 billion in 2008 and its cost per passenger per day has jumped from​ $10 to​ $33. Assuming that these firms are oligopolistic and the outcome is a Nash-Cournot equilibriumLOADING...​, why did prices rise less than in proportion to​ per-passenger-per-day cost? At the Cournot​ equilibrium, prices rose less than in proportion to​ per-passenger-per-day costs because A. . B. only fixed costs affect oligopoly prices. C. marginal costs do not affect oligopoly prices. D. . E. the number of firms must have decreased.