[sammyg2]

Wrong?
An arc is 3d.

A line is the shortest distance between two points in space with infinite points along it but has no real dimension. it does not curve, it is not a plane, it is a line. It is two dimensional.

If you started at the north pole and went straight out, you would be in space. Make a right turn and then another, you would still be in space.
Following the curvature of the earth is an arc and not a line.