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

$$\frac{gR}{R-x}$$

C

$$\frac{g^2}{R+x}$$

D

$$\left(\frac{gR^2}{R+x}\right)^{1/2}$$