Two blocks of masses $1\;kg$ and $2\;kg$ are connected by are connected by a metal wire going over a smooth pulley. The breaking stress of metal is $\large\frac{40}{3 \pi}$$\times 10^6\;Nm^{-2}$.What should be the minimum radius of wire used if it should not break? $(g=10\;m/s^{-2})$

Question

$$\begin {array} {1 1} (a)\;0.5\;mm & \quad (b)\;1\;mm \\ (c)\;1.5\;mm & \quad (d)\;2\;mm \end {array}$$