[求助]og11009

题目正确答案是：

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.

干扰项C为：

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

OG仅解释为：although一般要接含一个主语和一个动词的从句，但这里没有主语，sighted也不是一个完整的动词。

可是，although的从句不是可以省略主语和be动词吗？我认为，省略的两个条件——主语与主句主语相同、谓语是be——都满足呀。