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

1. A
   80 , \text{km}
2. B
   16 , \text{km}
3. C
   40 , \text{km}
4. D
   64 , \text{km}