$A$ balloon carrying some sand of mass $M$ is moving down with a constant acceleration $a_0$. The mass $m$ of sand to be removed so that the balloon moves up with double the acceleration $2a_0$ is:

• A
  $m=\frac{2 M a_0}{a_0+g}$
• B
  $m=\frac{2 M a_0}{a_0-g}$
• C
  $m=\frac{3 M a_0}{g+2 a_0}$
• D
  $m=\frac{3 M a_0}{g-2 a_0}$