A Tv transmitting antenna is $128$ m tall. If the reactivating antenna is at the ground level, the maximum distance between them for satisfactory communication is LOS mode is (R of earth $=6.4 \times 10^6 m)$

Question

$\begin{array}{1 1} 64 \sqrt {10} km \\ \frac{128}{\sqrt {10}} \\ 128 \sqrt{10} km \\ \frac{64}{\sqrt {10}}km \end{array}$