Parallels are:

• A. Great circles that pass through the poles.
• B. Circles on the Earth's surface whose planes pass through the center.
• C. Small circles on the Earth's surface parallel to the equator.
• D. The equator only.

Answer: (C)

Parallels are lines of latitude, created by slicing the Earth with planes that are parallel to the plane of the equator. Because these planes sit off-center, the resulting circles are smaller than the equator, so they are called small circles. The equator itself is a special case: its plane does pass through the Earth's center, making it a great circle. So parallels are small circles on the Earth's surface parallel to the equator, not circles whose planes pass through the center (except for the equator). And there isn’t just one parallel—the equator is the unique one that is a great circle.