Hi, tutor, I'd like to ask why this question choose the last one?

Question A13 The market for pens is perfectly competitive and is currently in equilibrium. What will happen if pens become more popular among university students? A) In the short run, firms will experience economic profits, but in the long run, firms will leave the market, bringing economic profits back down to zero. B) In the short run, firms will experience economic profits, but in the long run, firms will enter the market, bringing economic profits back up to zero. C) In the short run, firms will incur economic losses, but in the long run, firms will leave the market, bringing economic profits back down to zero. D) In the short run, firms will incur economic losses, but in the long run, firms will enter the market, bringing economic profits back down to zero. E) None of these answers are correct