关于although和but的用法..急死了。。请大家帮助

 了，现在做多了题目都不知道这有什么区别，请大家指教一下，谢谢了

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 X

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

(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 X（C）

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

这道题目，我选的是A，可是答案是C，我觉得首先这里BUT前面没有逗号，类似于没有逗号的and，那肯定就是连接2个主句中的从句。。句子结构sunspots are ....but have never been..

可是答案是C。。可是我想不通的是although前面没有加逗号，第二although后面省略了主语，直接就是过去分词的形式。。请问这是什么用法？这个sighted 是修饰主语的还是修饰就近的dark spots?