If over the course of a day, the maximum electron density in the ionosphere varies from $$10^{11}$$ to $$10^{12}$$ $$m^{-3}$$; the critical frequency changes approximately from:

A

2.2 MHz to 7 MHz

B

2.5 MHz to 8 MHz

C

2.8 MHz to 9 MHz

D

3.2 MHz to 10 MHz