A light wave with amplitude $A$ falls normally on a surface of area $S$. The power per unit area arriving at the surface is $P$. The amplitude of the light wave is then raised to $2A$. The light is afterwards focussed onto a smaller area $\frac{1}{3}S$. What is the power per unit area on this smaller area?
- A$36P$
- B$18P$
- C$12P$
- D$6P$