Infinitely many equidistant parallel lines are drawn in the plane. A positive integer $n \geqslant 3$ is called frameable if it is possible to draw a regular polygon with $n$ sides all whose vertices lie on these lines, and no line contains more than one vertex of the polygon.

(a) Show that $3, 4, 6$ are frameable.
(b) Show that any integer $n \geqslant 7$ is not frameable.
(c) Determine whether $5$ is frameable.

[i]Proposed by Muralidharan[/i]