$A$ satellite of mass $m$ revolves around the Earth of radius $R$ at a height $x$ from its surface. If $g$ is the acceleration due to gravity on the surface of the Earth,the orbital speed of the satellite is:

• A
  $gx$
• B
  ${\left( {\frac{{gR}}{{R - x}}} \right)^{1/2}}$
• C
  ${\left( {\frac{{gR^2}}{{R + x}}} \right)^{1/2}}$
• D
  ${\left( {\frac{{gR}}{{R + x}}} \right)^{1/2}}$