The value of acceleration due to gravity at Earth's surface is $9.8 \mathrm {~m} \mathrm {~s} ^ { - 2 }$. The altitude above its surface at which the acceleration due to gravity decreases to $4.9 \mathrm {~m} \mathrm {~s} ^ { - 2 }$, is close to: (Radius of earth $= 6.4 \times 10 ^ { 6 } \mathrm {~m}$)\\
(1) $1.6 \times 10 ^ { 6 } \mathrm {~m}$\\
(2) $2.6 \times 10 ^ { 6 } \mathrm {~m}$\\
(3) $6.4 \times 10 ^ { 6 } \mathrm {~m}$\\
(4) $9.0 \times 10 ^ { 6 } \mathrm {~m}$