$A$ block of mass $m$ moving with a velocity $v_0$ on a smooth horizontal surface strikes and compresses a spring of stiffness $k$ until the mass comes to rest,as shown in the figure. This phenomenon is observed by two observers:
$A$: standing on the horizontal surface
$B$: standing on the block
To an observer $A$,the work done by the spring force is

• A
  negative but nothing can be said about its magnitude
• B
  $ - \frac{1}{2}mv_0^2$
• C
  positive but nothing can be said about its magnitude
• D
  $ + \frac{1}{2}mv_0^2$