A yo-yo has a spool of mass $$m$$ and radius $$R$$. A massless string is wound around an axle of radius $$b$$ and of negligible mass. If the yo-yo released from rest has a downward acceleration of $$\frac{g}{9}$$, the ratio $$\frac{R}{b}$$ is

A

2

B

3

C

4

D

5