The value of acceleration due to gravity at Earth's surface is $9.8 \mathrm{~ms}^{-2}$. The altitude above its surface at which the acceleration due to gravity decreases to $4.9 \mathrm{~ms}^{-2}$, is close to:
(Radius of earth $=6.4 \times 10^6 \mathrm{~m}$ )