Final answer:
The intervals for relief vents in long mains depend on regional codes and standards, and without specifying these, it is not possible to provide an accurate answer to the question.
Step-by-step explanation:
The question pertains to the intervals at which relief vents should be provided on long mains. The specific intervals needed for placing relief vents can vary based on the codes and standards applicable to plumbing and piping systems in a particular region or country. In absence of the provided choices matching those found in common standards, and without clarity on the jurisdiction or applicable standard, it is not possible to confidently give an accurate answer to this question.
Relief venting is a critical design element in plumbing to prevent negative pressure or vacuum conditions which could lead to siphonage, affecting trap seals and hence, the performance of the drainage system.