The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be ${\mathrm{f}}_{\max } = 9 ({\mathrm{N}}_{\max }{)}^{1/2}$, where ${\mathrm{N}}_{\max }$ is the maximum electron density at that layer of the ionosphere. On a certain day, it is observed that signals of frequencies higher than $8 \mathrm{MHz}$ are not received by reflection from the ${\mathrm{F}}_2$ layer of the ionosphere. Estimate the maximum electron densities of the ${\mathrm{F}}_2$ layers on that day.

• Correct answer
  A

  $7.9\times {10}^{11} {\mathrm{m}}^{-3}$
• B

  $8.9\times {10}^{11} {\mathrm{m}}^{-3}$
• C

  $1.9\times {10}^{11} {\mathrm{m}}^{-3}$
• D

  $7.1\times {10}^{11} {\mathrm{m}}^{-3}$

Hence, the correct answer is option: 'A'.