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# 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.

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

Since, the waves would not be reflected from the ionosphere, so it cannot be used for communication.
Ans : (a)