Concept:Firms are more likely to collude to maximise joint profit when new firms cannot easily enter and compete away the profit.Explanation:Joint profit maximisation usually requires firms to agree on output and price, often by forming a cartel or colluding.If there are significant barriers to entry, existing firms can keep higher profits without fear of new competitors entering and disrupting the agreement.The other options — many differentiated products, rapid technological change, and a large number of producers — make collusion more difficult.Answer:D. There are significant barriers to prevent firms entering the market.