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A rectangular waveguide having $\mathrm{TE}_{10}$ mode as dominant mode is having a cutoff frequency of $18-\mathrm{GHz}$ for the $\mathrm{TE}_{w,}$ mode. The inner broad-wall dimension of the rectangular waveguide is

1. $\frac{5}{3} \; \mathrm{cms}$
2. $5 \; \mathrm{cms}$
3. $\frac{5}{2} \; \mathrm{cms}$
4. $10 \; \mathrm{cms}$