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
- $6.25 \mathrm{~GHz}$
- $6.0 \mathrm{~GHz}$
- $5.0 \mathrm{~GHz}$
- $3.75 \mathrm{~GHz}$