$A$ vertical closed cylinder is separated into two parts by a frictionless piston of mass $m$ and of negligible thickness. The piston is free to move along the length of the cylinder. The length of the cylinder above the piston is $l_1,$ and that below the piston is $l_2,$ such that $l_1 > l_2.$ Each part of the cylinder contains $n$ moles of an ideal gas at equal temperature $T.$ If the piston is stationary,its mass $m$ will be given by: ($R$ is the universal gas constant and $g$ is the acceleration due to gravity)

• A
  $\frac{RT}{ng} \left[ \frac{l_1 - 3l_2}{l_1 l_2} \right]$
• B
  $\frac{RT}{g} \left[ \frac{2l_1 + l_2}{l_1 l_2} \right]$
• C
  $\frac{nRT}{ng} \left[ \frac{1}{l_2} + \frac{1}{l_1} \right]$
• D
  $\frac{nRT}{g} \left[ \frac{l_1 - l_2}{l_1 l_2} \right]$