A horizontal line is one that is:

• A. Perpendicular to the direction of gravity at a point.
• B. Parallel to the direction of gravity at a point.
• C. Perpendicular to the equator at a point.
• D. Parallel to the equator at a point.

Answer: (A)

A horizontal line is defined by its relation to gravity: it lies in the plane that is perpendicular to the local direction of gravity (the vertical). At any point on Earth, gravity pulls toward the center, so the horizontal direction is any line that does not tilt with respect to that vertical and remains level as you move along it. This is the foundation used in surveying to describe level lines and true horizontals. The other descriptions describe directions relative to gravity or to the Earth’s surface in ways that don’t match the standard horizontal definition: a line parallel to gravity would point vertically, not horizontally; a line perpendicular to the equator points toward the poles and isn’t the basis for the horizontal plane; and while a line that runs east–west is common in practice, the defining criterion for horizontality is perpendicularity to gravity, not its relation to the equator.