Initially the spring is undeformed. Now the force \[F\] is applied to \[B\] as shown in the figure. When the displacement of \[B\] w.r.t. \[A\] is x towards right in some time then the relative acceleration of \[B\] w.r.t. \[A\] at that moment is

• A. \[\frac{F}{2m}\]                       
• B. \[\frac{F-kx}{m}\]
• C. \[\frac{F-2kx}{m}\]      
• D. None of these