If the firm is a "price discriminating" monopolist, then it has the capacity to extract more resources from the consumer. It charges a lump sum fee, as well as a per unit cost. In order to sell the maximum number of units, the firm must charge the perfectly competitive price per unit, Pc, because this is the only price at which Qc units can be sold (note this is also the marginal cost per unit). To make up for the lower cost per unit, the firm then imposes a fee upon our consumer equal to her consumer surplus, ABC.