S1: Electromagnetic waves with frequencies more than the critical frequency of ionosphere cannot be used for communication using sky wave communication. S2: The refractive index of ionosphere becomes very high for frequencies higher than the critical frequency.

Question

$\begin {array} {1 1} (a)\;\text{A. S1 and S2 are true; S2 is a correct explanation of S1.}\\ (b)\;\text{S1 and S2 are true; S2 is not a correct explanation of S1.} \\ (c)\;\text{S1 is true; S2 is false} \\ (d)\;\text{S1 is false; S2 is true.} \end {array}$