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（A） (E) appear as dark spots on the Sun’s surface, which have never been sighted on
偶对答案A没有异议，也做对了。但是看OG关于C \ D的解释(如下)，发现although不能省略后面的主谓，就算这题里从句主语显然就是主句的主语： C.Although typically introduces a subordinate clause, which has a subject and a verb, but here there is no subject and sighted is not a complete verb
D.Although usually introduce a subordinate clause, but there is no subject of the clause and having been sighted is not a complete verb phrase 这好像和刘振民和白勇的说法有些出入。如果说OG就用这个就排除了，这应该是一个correctness的错误吧。 刘振民说：如果although/when/if... 之类的引导的状语从句，后面从句主语如果和主句一致，且同时谓语动词是be， 就可以省略主谓。
是否可以这样理解： 以上构成省略的be指的是实义动词be， 而不是助动词be，而C\D选项中 sighted 和 having been sighted 就算前面有个be省略，那个be也是助动词，所以不能省略，所以错。（何况be having been sighted也绝对awkward）