Let's say "c" is a constant, hmmmm any constant, for any value whatsoever of "x", "y" is always that constant, for example, say c = 3, thus y = 3, so a table for it will look like

now, if you plot those points, it'd looks like the picture below.
This answer is a (A) segment AD is an altitude at angle ABC
radius is the distance from the centre of a circle to any given point on its circumference, diameter is the length between two points that are the furthest possible from each other on the circle's circumference.
The diameter is twice the radius.
Therefore, if the radius is 12in then the diameter is 24in.