A spool of mass `M` and radius `2R` lies on an inclined plane as shown in the figure. A light thread is wound around the connecting tube of the spool and its free end carries a weight of mass `m`. The value of `m` so that system is in equilibrium is
A. `2M sin alpha`
B. `M sin alpha`
C. `2M tan alpha`
D. `M cos alpha`