A radar has a power of 1 Kw and is operating at a frequency of 10 GHz. It is located on a mountain top of height 500 m. The maximum distance upto which it can detect object located on the surface of the earth (Radius of earth = $6.4 \times 10^{6} m$ ) is

Question

( A ) 40 km
( B ) 16 km
( C ) 80 km
( D ) 64 km