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At $20 \; \mathrm{GHz}$, the gain of a parabolic dish antenna of $1$ meter diameter and $70 \%$ efficiency is

1. $15 \mathrm{~dB}$
2. $25 \mathrm{~dB}$
3. $35 \mathrm{~dB}$
4. $45 \mathrm{~dB}$