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A rectangular waveguide of internal dimensions ( $\mathrm{a}=4 \mathrm{~cm}$ and $\mathrm{b}=3 \mathrm{~cm}$ ) is to be operated in $\mathrm{TE}_{11}$ mode. The minimum operating frequency is

1. $6.25 \mathrm{GHz}$
2. $6.0 \mathrm{GHz}$
3. $5.0 \mathrm{GHz}$
4. $3.75 \mathrm{GHz}$