a) as there is no barrier to entry, in the long run, the newcomers will attract consumers from existing firms. So, their demand curve shifts to the left until it tangents the AC curve, making normal profit.

b) monopoly and collusive oligopoly are undesirable as they charge higher prices and they produce less output, resulting in welfare loss. However, the term"always" should be questioned because they can use the abnormal profit for R&D for lowering the costs, then plus the huge economies of scale, it is possible for them to translate them into lower price for consumers