In the Equation $\vec { M } + \vec { v } = \vec { P } + \vec { Y } _ { R }$

In the equation $\vec { M } + \vec { v } = \vec { P } + \vec { Y } _ { R }$ , what does $\bar { M }$
Stand for?

A) the money supply
B) growth in the money supply
C) money velocity
D) growth in money velocity