A radar has a power of $1 \mathrm{~kW}$ and is operating at a frequency of $10 \mathrm{GHz}$. It is located on a mountain top of height $500 \mathrm{~m}$. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth $=6.4 \times 10^{6} \mathrm{~m}$ ) is [AIEEE 2012]
(a) $80 \mathrm{~km}$
(b) $16 \mathrm{~km}$
(c) $40 \mathrm{~km}$
(d) $64 \mathrm{~km}$