Two disks $A$ and $B$ of mass $m_a = 1$ $kg$ and $m_b = 4$ $kg$ are connected by a belt. If a couple $M$ is applied to disk $A$ and assuming no slipping, the angular acceleration are related by

A. $\alpha_a = 4\alpha_b$
B. $\alpha_a = 3\alpha_b$
C. $\alpha_a = 12\alpha_b$
D. $\alpha_a = \frac{3}{4}\alpha_b$