If a spring extends by x on loading, then energy stored by the spring is (if T is the tension in the spring and K is the spring constant) 

• A

  $\frac{{\mathrm{T}}^2}{2\mathrm{x}}$

• B

  $\frac{{\mathrm{T}}^2}{2\mathrm{K}}$

• C

  $\frac{2\mathrm{K}}{{\mathrm{T}}^2}$

• D

  $\frac{2{\mathrm{T}}^2}{\mathrm{K}}$