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

• A.     the refractive index of the ionosphere becomes very high for \[f>{{f}_{c}}\]
• B. the refractive index of the ionosphere becomes very low for \[f>{{f}_{c}}\]
• C. the refractive index of the ionosphere very high for \[f<{{f}_{c}}\]
• D. none of the above