A nearly massless rod is pivoted at one end so that it can swing freely as a pendulum. Two masses 2m and m are attached to it at distance b and 3b respectively, from the pivot. The rod is held horizontal and then released. Find its angular acceleration at the instant it is released.

• A

  $\frac{2\mathrm{g}}{11\mathrm{b}}$

• B

  $\frac{4\mathrm{g}}{17\mathrm{b}}$

• C

  $\frac{5\mathrm{g}}{11\mathrm{b}}$

• D

  $\frac{5\mathrm{g}}{12\mathrm{b}}$