Spirit level bubble on a marble shelf tests for horizontality
In physics, engineering and construction, the direction designated as vertical is usually that along which a plumb-bob hangs. Alternatively, a spirit level that exploits the buoyancy of an air bubble and its tendency to go vertically upwards may be used to test for horizontality. A water level device may also be used to establish horizontality.
Modern rotary laser levels that can level themselves automatically are robust sophisticated instruments and work on the same fundamental principle.[4][5]
The flat earth approximation
In the flat earth scenario,[6] where the earth is notionally a large (infinite) flat surface with gravitational field at a right angle to the surface, the earth surface is horizontal and any plane parallel to the earth surface is also horizontal. Vertical planes, e.g., walls, may be parallel to each other or they may intersect at a vertical line. Horizontal surfaces do not intersect. Furthermore, a plane cannot both be a horizontal plane at one place and a vertical plane somewhere else.
Vertical & Horizontal Lines
The spherical Earth
Strictly, vertical walls are never parallel on the surface of a spherical planet
When the curvature of the earth is taken into account, the concepts of vertical and horizontal take on yet another meaning. On the surface of a smoothly spherical, homogenous, non-rotating planet, the plumb bob picks out as vertical the radial direction. Strictly speaking, it is now no longer possible for vertical walls to be parallel: all verticals intersect. This fact has real practical applications in construction and civil engineering, e.g., the tops of the towers of a suspension bridge are further apart than at the bottom. [7]
On a spherical planet, horizontal planes intersect. In the example shown, the blue line represents the tangent plane at the North pole, the red the tangent plane at an equatorial point. The two intersect at a right angle.
Also, horizontal planes can intersect when they are tangent planes to separated points on the surface of the earth. In particular, a plane tangent to a point on the equator intersects the plane tangent to the North Pole at a right angle. (See diagram). Furthermore, the equatorial plane is parallel to the tangent plane at the North Pole and as such has claim to be a horizontal plane. But it is. at the same time, a vertical plane for points on the equator. In this sense, a plane can, arguably, be both horizontal and vertical, horizontal at one place, and vertical at another.
Step-by-step explanation:
So the answer is C or B
