677. Sunspots, vortices of gas associated with strong electromagnetic
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
(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
答案是C。可是为什么是although never sighted at, 偶觉得应该是although
never been sighted at才对啊 | 
