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

sighted on 答案是C。可是为什么是although never sighted at, 偶觉得应该是although

never been sighted at才对啊

当条件状语的主语和主句的主语一致时，条件状语的主语和be动词可以一

起省略。所以C的结构是可以接受的；此外，虽然at/on the quatorat都可以使用

，但at the poles是比较合适的介词结构；所以用at替代所有的介词，可行；另

外，are visible as dark spots on the surface中on the surface修饰dark

spots不合适，事实上，太阳表面并没有dark spots，而只是我们看到的好像有

dark spots；