The marginal cost is equal is to marginal revenue, the average cost is equal to average revenue, average revenue is equal to marginal revenue, and the average cost is equal to marginal cost. This is the condition of
1. Long-period equilibrium for a firm under monopoly.
2. Short-period equilibrium for a firm under oligopoly.
3. Long-period equilibrium
4. Long-period equilibrium for a firm under perfect competition.