Microecon question on perfect and monopolistic competition...??

Question

1. Why are long run profits not possible under conditions of perfect competition ( and monopolistic competition)?

2. Why are long run profits possible under conditions of monopoly (and oligopoly)??

Answer 1

1: Because of entry of new firms which are attracted by profits, thus supply increases and price falls till level where profits=0, so profits are eliminated.

2: Because of "free-entry" limitations.