Which of the following is incorrect under a perfectly competitive market?a$.$ a firm's demand curve is perfectly elasticb. a firm's demand curve equals to AR $=$ MR $=$ Pc$.$ a firm's demand curve is downward sloping while market's demand curve is downward slopingd. zero economic profits in the long run, even though economic profits or losses could be possible in the short rune. market's supply curve increases as firms start entering the market in response to positive economic profits