Two masses $$A$$ and $$B$$, each of mass $$M$$ are fixed together by a massless spring. A force acts on the mass $$B$$ as shown in figure. If the mass $$A$$ starts moving away from mass $$B$$ with acceleration $$a$$, then the acceleration of mass $$B$$ will be:

A

$$\frac{MF}{F + Ma}$$

B

$$\frac{F + Ma}{M}$$

C

$$\frac{Ma - F}{M}$$

D

$$\frac{F - Ma}{M}$$