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A radio wave is incident on a layer of ionosphere at an angle of $30$ degree with the vertical. If the critical frequency is $1.2 \mathrm{MHz}$, the maximum usable frequency is

1. $1.2 \mathrm{MHz}$
2. $2.4 \mathrm{MHz}$
3. $0.6 \mathrm{MHz}$
4. $1.386 \mathrm{MHz}$