Monochromatic light of wavelength $667 \, nm$ is produced by a helium-neon laser. The power emitted is $9 \, mW$. The number of photons arriving per second on average at a target irradiated by this beam is:

  • A
    $3 \times 10^{16}$
  • B
    $9 \times 10^{15}$
  • C
    $3 \times 10^{19}$
  • D
    $9 \times 10^{17}$

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