A transmitting antenna is kept on the surface of the earth. The minimum height of receiving antenna required to receive the signal in line of sight at 4 km distance from it is $x \times 10^{-2}$ m. The value of $x$ is $\_\_\_\_$. (Let, radius of earth $R = 6400$ km)\\
(1) 125\\
(2) 1250\\
(3) 12.5\\
(4) 1.25