The equator is:

• A. A great circle and also a parallel of latitude.
• B. A circle that lies entirely within the Earth.
• C. A line of longitude.
• D. A great circle that is also a parallel of latitude.

Answer: (A)

The equator is a circle that lies on the Earth's surface and runs around its middle. It is a great circle because the plane that creates it passes through the Earth's center, giving the largest possible circle on the sphere. It is also a parallel of latitude at 0°, meaning it is one of the lines used to measure latitude, and among these, it is the special one that is a true great circle. It is not a line of longitude, since meridians run from pole to pole and are not circles around the globe. It isn’t a circle located inside the Earth; it sits on the surface where the sphere is cut by the central plane.