Two blocks of masses M and m are connected to each other by a massless string and spring of force constant k as shown in the figure. The spring passes over a frictionless pulley connected rigidly to the edge of a stationary block A. The coefficient of friction between block M and the plane horizontal surface of A is $\mu$. The block M slides over the horizontal top surface of A and block m slides vertically downwards with the same speed. The mass M is equal to

• A

  $\frac{m}{\mu }$

• B

  $\frac{2m}{\mu }$

• C

  $m\mu$

• D

  $\mu mg$