When a Perfectly Competitive Firm (That Sells Its Good for $20

When a perfectly competitive firm (that sells its good for $20 per unit) hires 1 unit of factor X it produces 70 units of output and when it hires 2 units of factor X it produces 85 units of output. Marginal revenue product of the second unit of factor X is equal to

A) $30.
B) $530.
C) $265.
D) $300.