The Exposure Coefficient in the Regression $P = a + b \times S + e$

The exposure coefficient in the regression $P = a + b \times S + e$ is given by:

A) $b = \frac { \operatorname { Cov } ( P , S ) } { \operatorname { Var } ( S ) }$
B) $P = a + b \times S + e$
C) $b = \frac { \operatorname { Cov } ( P , S ) } { \operatorname { Var } ( P ) }$