# Light with an energy flux of $18\: W/cm^2$ surface has an area of $20\: cm^2$ falls on a non-reflecting surface at normal incidence. If the , the average force exerted on the surface during a $30$ minute time span is
$\begin {array} {1 1} (a)\;0.12 \times 10^{-6}N & \quad (b)\;12 \times 10^{-6}N \\ (c)\;12 \times 10^{-8}N & \quad (d)\;1.2 \times 10^{-6}N \end {array}$