Q1: Sunspots, vortices of gas associated with strong eleconmagnetic activity, are visible as dark spots on the surface of the Sun but have never been sighted on the Sun’s poles or Equator.

(A)   are visible as dark spots on the surface of the Sun but have never been sighted on the

(B)   are visible as dark spots that never have been sighted on the surface of the Sun

(C)   appear on the surface of the Sun as dark spots although never sighted at

(D)  appear as dark spots on the surface of the Sun, although never having been sighted at

(E)   appear as dark spots on the Sun’s surface, which have never been sighted on

which one is the correct anwser and why?

thanks

选c首先用at poles and equator,不用on

此外done 优于having been done

A is best.

Note this a 'but' in answer A. This emphasizes the last part of this answer which isn't reflected by other answers.