A two-way cleanout is not required for a dwelling if the developed length between the first cleanout and the city main is _____________ feet or less.

The correct choice indicates that a two-way cleanout is not required for a dwelling if the developed length between the first cleanout and the city main is 120 feet or less. This regulation is in place to ensure that plumbing systems are efficient and manageable, without unnecessary structural requirements.

In practical terms, the reasoning behind this distance is related to the ability to effectively clean and maintain the sewer line without needing to access it from both directions. If the developed length exceeds 120 feet, it's often deemed necessary to provide a two-way cleanout to facilitate easier access and maintenance in case of blockages or other issues that might arise.

Therefore, if the distance is within this limit, it simplifies the plumbing design and reduces costs while still complying with safety and maintenance standards outlined by plumbing codes.