$A$ block of mass $M$ is being pulled along a rough horizontal surface. The coefficient of friction between the block and the surface is $\mu$. If another block of mass $M/2$ is placed on the block and it is again pulled on the surface,the coefficient of friction between the block and the surface will be

• A
  $\mu$
• B
  $3\mu / 2$
• C
  $2\mu$
• D
  $5\mu / 2$