Concept:Any pair of opposite longitudes together forms a complete Great Circle around the Earth.
Explanation:Each longitude is a semicircle that runs from the North Pole to the South Pole.
The meridian exactly opposite to a given longitude lies
180∘ away.
When these two opposite meridians are combined, they form one full circle.
This full circle passes through the poles and has its centre at the Earth’s centre.
A circle whose centre is the Earth’s centre is called a Great Circle.
So two opposite longitudes always make a Great Circle, not the Equator or the Arctic Circle.
Answer:A. a Great Circle