Which statement best contrasts great circles with parallels?

• A. Great circles are formed by planes through Earth's center; parallels are lines of latitude parallel to the equator.
• B. Great circles are only at the equator; parallels exist elsewhere.
• C. Parallels are formed by planes perpendicular to the equator.
• D. The equator is never a great circle.

Answer: (A)

The essential idea is that a great circle comes from a plane that cuts through Earth’s center, while parallels come from planes that are parallel to the equatorial plane and do not pass through the center. Because the equator lies in a plane that does pass through the center, it is a great circle. Any other parallel sits in a plane offset from the center, so it forms a smaller circle and is not a great circle. So the statement is correct because it cleanly contrasts the central-plane requirement of great circles with the offset-plane nature of parallels. The equator itself is a great circle, which is why saying it is never a great circle is false. Great circles exist in many positions, not only at the equator, and parallels are not defined by planes perpendicular to the equator but by planes parallel to it.