Which pair of undefined terms is used to define a ray?
a. line and plane
b. plane and line segment
c. point and line segment
d. point and line

The answer is "D. Point and line".

Point is the fundamental building square of Geometry. Each shape is made through the joining the focuses. A little spot set apart by a pencil is a point. A point has no length or width. It has no thickness. Point is a sign of position. A point indicates the correct area.

A line abandons end in both directions. The two closures of a line can go to unendingness. A line has no end focuses. The length of a line can't be estimated. A line has no clear length.