CR蓝皮书的几道语法题，请大家赐教呀

Sunspots , vortices of gas associated with strong electro-magnetic activity, are visible as dark spots on the surface of the Sun but have never been sighted on the Sun's poles or equator.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 never been sighted on
这道题做了至少有四遍了，但每次都错，而且错的都是不一样的答案，不明白为什么要选用介词AT而不是ON，还是说这道题跟选介词哪个没关系，为什么不能用ARE VISIBLE 而选了APPEAR，同样不明白
答案是C