If $g$ is the acceleration due to gravity on the earth's surface, the gain in the potential energy of object of mass $m$ raised from the surface of the earth to a height equal to the radius $R$ of the earth is\\
(1) 2 mgR\\
(2) $\frac { 1 } { 2 } \mathrm { mgR }$\\
(3) $\frac { 1 } { 4 } \mathrm { mgR }$\\
(4) mgR