A Monopolist Faces an Inverse Demand Curve $P = 300 - 6 Q$

A monopolist faces an inverse demand curve $P = 300 - 6 Q$ and has a constant marginal cost of 20. The IEPR formula for this monopolist could be stated in the following way:

A) $\frac { P - 20 } { P } = \left( \frac { 1 } { P / 6 } \right)$
B) $\frac { P - 20 } { P } = \frac { 300 - P } { P }$
C) $\frac { P - 20 } { P } = \frac { P } { 300 - P }$
D) $\frac { P - 20 } { P } = - P ( Q / 6 )$