A block attached to a spring, pulled by a constant horizontal force, is kept on a smooth surface as shown in figure. Initially, the spring is in the natural length state. Then the maximum positive work that the applied force F can do is (given that spring does not break)

• A

  $\frac{{\mathrm{F}}^2}{\mathrm{k}}$

• B

  $\frac{2{\mathrm{F}}^2}{\mathrm{k}}$

• C

  $\infty$

• D

  $\frac{{\mathrm{F}}^2}{2\mathrm{k}}$