A. What is an interval of a 7th?

Explanation:

In classical music from Western culture, a seventh is a musical interval encompassing seven staff positions (see Interval number for more details), and the major seventh is one of two commonly occurring sevenths. It is qualified as major because it is the larger of the two.

No idea if you mean by music, but as in math:

An interval is a range of numbers between two given numbers and includes all of the real numbers between those two numbers. ... Intervals can be written using inequalities, a number line, or in interval notation!

It depends on what is used.

The whole note one the third line

Explanation: you take the whole note and start counting line then space till you get to seven. Hope this helps:)

i believe their role is to instruct certain sections on when they are to join and when they are to stop