Light falls on a non-reflecting surface normally. If the average force exerted on a surface with area $15 \text{ cm}^2$ during a $20 \text{ minute}$ time interval is $10^{-6} \text{ N}$,then the energy flux (intensity) of light is (Velocity of light $= 3 \times 10^8 \text{ ms}^{-1}$)

• A
  $20 \times 10^4 \text{ Wm}^{-2}$
• B
  $15 \times 10^4 \text{ Wm}^{-2}$
• C
  $25 \times 10^4 \text{ Wm}^{-2}$
• D
  $10 \times 10^4 \text{ Wm}^{-2}$