Two mass A and B each of mass M are fixed together by a massless spring A force F acts on the mass B as shown in figure. At the instant shown, the mass A has acceleration a. What is the acceleration of mass B ?

• A.  \[\left( \frac{F}{M} \right)-a\]    
• B.         \[a\]
• C.  \[-a\]                   
• D.         \[\left( \frac{F}{M} \right)\]