$A$ particle of mass $m$ is kept at rest at a height $3R$ from the surface of the Earth,where $R$ is the radius of the Earth and $M$ is the mass of the Earth. The minimum speed with which it should be projected upward so that it does not return back is ($g$ = acceleration due to gravity on the Earth's surface).

• A
  $[\frac{GM}{2R}]^{1/2}$
• B
  $[\frac{gR}{4}]^{1/2}$
• C
  $[\frac{2g}{R}]^{1/2}$
• D
  $[\frac{GM}{R}]^{1/2}$