A spring elongated by length l when a mass 'M' is suspended to it. Now a small mass 'm' is attached and then released, its time period of oscillation is:
 

• A

  $2\mathrm{\pi }\sqrt{\frac{\left(\mathrm{M}+\mathrm{m}\right)\mathrm{l}}{\mathrm{Mg}}}$

• B

  $2\mathrm{\pi }\sqrt{\frac{\mathrm{ml}}{\mathrm{Mg}}}$

• C

  $2\mathrm{\pi }\sqrt{\frac{\mathrm{l}}{\mathrm{g}}}$

• D

  $2\mathrm{\pi }\sqrt{\frac{\mathrm{Ml}}{\left(\mathrm{M}+\mathrm{m}\right)\mathrm{g}}}$