Two towers on top of two hills are 40 km apart. The line joining then passes 50 m above a hill halfway between the tower . What is the longest wavelength of radio waves , which can be sent between the towers with out appreciable diffraction effects ?

Question

$\begin{array}{1 1} 0.125\;m \\ 1.125\;m \\12.5 \;m \\ 125\;m \end{array} $