Question

Diametrically opposite lines of longitude and the equator are called great circles
Give reason

Answer 1

A great circle is any circle on the surface of a sphere that has it's center at the center of the sphere. The equator is only parallel of latitude that's a great circle. Every meridian of constant longitude is a semi circle, and together with the one directly opposite it, they form a great circle.
The shortest distance between two points on a sphere is the piece of the great circle through them.