Which of the Following Statements About Marginal Revenue for a Perfectly

Which of the following statements about marginal revenue for a perfectly competitive firm is incorrect, where TR stands for total revenue, P stands for price, and q stands for output?

A) $M R = \frac { \Delta T R } { q }$
B) $M R = P$
C) $M R = \frac { \Delta T R } { \Delta Q }$
D) Marginal revenue is the rate at which total revenue changes with respect to changes in output.