A spring of force constant ‘K’ is stretched by a force ‘F’. The energy stored in  the spring is

• A

  $\frac{{\text{F}}^{\text{2}}}{\text{K}}$

• B

  $\frac{{\text{F}}^{\text{2}}}{\text{2K}}$

• C

  ${\text{F}}^{\text{2}}\text{.K}$

• D

  $\text{F.K}$