A spring with spring constant k is extended from $\mathrm{x}=0$ to $\mathrm{x}={\mathrm{x}}_1$. The work done will be  

• A

  ${\mathrm{kx}}_1^2$

• B

  $\frac{1}{2}{\mathrm{kx}}_1^2$

• C

  $2{\mathrm{kx}}_1^2$

• D

  $2{\mathrm{kx}}_1$