The figure shows a two-block system. $A$ $4 \,kg$ block rests on a smooth horizontal surface,and the upper surface of the $4 \,kg$ block is rough. $A$ block of mass $2 \,kg$ is placed on its upper surface. Given $\mu_s = 0.8$ and $\mu_k = 0.6$ between the two blocks. The acceleration of the upper block with respect to the earth when the $4 \,kg$ mass is pulled by a force of $30 \,N$ is ......... $m/s^2$.

• A
  $6$
• B
  $5$
• C
  $8$
• D
  $2$