To ensure that only dominant mode TE₁₀ is allowed to propagate in an air filled rectangular waveguide, the lower frequency limit and upper frequency limit are

• A. about 25% above fc for TE₁₀ and 5% below fc for TE₂₀ mode respectively
• B. about 50% above fc for TE₁₀ and 25% below fc for TE₂₀ mode respectively
• C. about 25% above fc for TE₁₀ and 25% below fc for TE₂₀ mode respectively
• D. none of the above