What determines entry and exit of firms in a perfectly competitive industry in the long run? in a perfectly competitive industry in the long run, a. new firms will enter if existing firms are making a profit and existing firms will exit if they are experiencing losses. b. new firms will enter if market demand exceeds market supply and existing firms will exit if market supply exceeds market demand. c. new firms cannot enter the market due to barriers but existing firms will exit if they are experiencing losses. d. new firms will enter if existing firms are making a profit and existing firms will exit if they are breaking even or experiencing losses. e. new firms will enter if price is above the shutdown point and existing firms will exit if price is below the shutdown point?