In the given figure, the mass of blocks $A$ and $B$ are $m_{1}$ and $m_{2}$ respectively, the pulley is circular disc of mass $m$ and radius $r$. The pulley is free to rotate about $O$. No friction exists between $A$ and $B$ and the floor. The magnitude of acceleration of block $B$ is $a_{0}$. Then the magnitude of acceleration of $A$ is :
(a) $a_{0}$
(b) $2 a_{0}$
(c) $\sqrt{2} a_{0}$
(d) $g$