$A$ light with an energy flux of $18 \,W \,cm^{-2}$ falls on a non-reflecting surface at normal incidence. If the surface has an area of $20 \,cm^2$, the average force exerted on the surface during a $30$ minute time span is . . . . . . $\left(c=3 \times 10^8 \,ms^{-1}\right)$.

• A
  $1.2 \times 10^{-6} \,N$
• B
  $2.1 \times 10^{-6} \,N$
• C
  $3.1 \times 10^{-6} \,N$
• D
  $4.8 \times 10^{-6} \,N$