When a mass M is attached to the spring of force constant k, then the spring stretches by l. If the mass oscillates with amplitude l, what will be maximum potential energy stored in the spring 

• A

  $\frac{\mathrm{kl}}{2}$

• B

  $2\mathrm{kl}$

• C

  $\frac{1}{2}\mathrm{Mgl}$

• D

  Mgl