Two blocks of masses 1 kg and 2 k8 are connected by a metal wire going over a smooth pulley as shown in fig. The breaking stress of the metal is 

 

$2\times {10}^9\mathrm{N}/{\mathrm{m}}^2$.If the wire is not to break, its minimum radius should be

• A

  $4\cdot 6\times {10}^{-5}\mathrm{m}$

• B

  $6\cdot 4\times {10}^{-5}\mathrm{m}$

• C

  $4\cdot 6\times {10}^{-6}\mathrm{m}$

• D

  $6\cdot 4\times {10}^{-6}\mathrm{m}$