What type of fittings are allowed for connections of vents and drains?

The correct answer is focused on the importance of maintaining proper flow in plumbing systems. Approved fittings that are specifically designed and tested for venting and draining applications ensure that the entire plumbing system functions optimally. This is crucial not only for effective drainage but also to prevent issues such as backflow or improper venting, which can lead to pipe blockages or sewer gas entering the living space.

In the context of plumbing codes, using standard fittings is not sufficient if those fittings do not meet specific criteria for water flow and venting. Non-metallic fittings may have their applications, but they are not universally accepted for all types of drainage and venting scenarios. Lastly, while ease of installation is a practical consideration, it should not outweigh the necessity of using fittings that are approved and maintain the integrity of the plumbing system's performance. Therefore, the emphasis on flow in the correct answer highlights the critical nature of choosing the right fittings based on code requirements and functional considerations.