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

• about 25% above fc for TE10 and 5% below fc for TE20 mode respectively
• about 50% above fc for TE10 and 25% below fc for TE20 mode respectively
• about 25% above fc for TE10 and 25% below fc for TE20 mode respectively
• none of the above