A Friend Hands You an Equation Sheet with the Following $f ^ { \prime } = \frac { \left( v - v _ { 0 } \right) } { \left( v + v _ { s } \right) } f$

A friend hands you an equation sheet with the following equation for the Doppler effect: $f ^ { \prime } = \frac { \left( v - v _ { 0 } \right) } { \left( v + v _ { s } \right) } f$ .This version of the equation is correct with signs as given only if

A) the observer and source are approaching each other.
B) the observer is approaching the source while the source is moving away from the observer.
C) the observer is moving away from the source while the source is approaching the observer.
D) the observer and source are moving away from each other.
E) the observer and source are moving in perpendicular directions.