$A$ block of mass $M$ is placed on a horizontal surface and is tied with an inextensible string to a block of mass $m$,as shown in the figure. $A$ block of mass $m_0$ is also placed on $M$. If the surface between $M$ and $m_0$ has a coefficient of friction $\mu$,find the tension in the string,assuming the system moves together.

• A
  $\frac{mM}{m + M}g$
• B
  $\frac{m(m_0 + M)}{m + m_0 + M}g$
• C
  $\left[ \frac{m + \mu(m_0 + M)}{m + M} \right]Mg$
• D
  $\left[ \frac{mM + \mu m(m_0 + M)}{m + M} \right]g$