Use Figure: Demand for Gasoline

Use Figure: Demand for Gasoline. The figure shows the demand curve for gasoline in a small town that has only one provider, which therefore is a monopolist. What is the marginal revenue for the monopolist from increasing its sales from two gallons to three gallons?
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Figure: Demand for Gasoline

A) $1.00 per gallon
B) $1.50 per gallon
C) $2.00 per gallon
D) $2.50 per gallon