Which definition describes best the notion “poles”?
选项：

A: The poles are the points of intersection between the surface of the earth and the extended line joining the star Polaris with the centre of the earth
B: The poles are the points of intersection between the earth's axis and the surface of the earth
C: The poles are the points on the surface of the earth where gravity acts under an angle of 90°
D: The poles are the points on the surface of the earth which have the same distance to all points of the eliptic