An important spectral emission line has a wavelength of $21 \ cm$. The corresponding photon energy is $(h = 6.62 \times 10^{-34} \ J \cdot s; \ c = 3 \times 10^8 \ m/s)$.

  • A
    $5.9 \times 10^{-4} \ eV$
  • B
    $5.9 \times 10^{-6} \ eV$
  • C
    $5.9 \times 10^{-8} \ eV$
  • D
    $11.8 \times 10^{-6} \ eV$

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