Electromagnetic waves with frequency greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation, because:

Options

A. the refractive index of the ionosphere becomes very high `fgtf_(c)`
B. the refractive index of the ionosphere very high low `fgtf_(c)`
C. the refreactive index of the ionosphere very high for `fgtf_(c)`
D. None of these

Answer

a

Explanation

For total of the ionosphere should be less. If `fgtf_(c)'` the refractive index will increase.