Sunspots, vortices of gas associated with strong electromagnetic activity, appear on the surface of the Sun as dark spots although never sighted at the Sun’s poles or equator.
(C) appear on the surface of the Sun as dark spots although never sighted at 是不是因为主句和从句虽然共用一个主语,但是谓语动词不同,一个appear,另一个have been所以不能省略呢?如果把appear改为are visible呢?