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（C）

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

不懂为什么选C

解释说C
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

记得although、though等后面主谓可以省，如果此处为although (they are) sighted...

不明白的是为何用完成时，如果非用完成时，那么although后就不可以省

C和A在意义上有很大区别：

C实际上不构成对比，先说太阳黑子出现太阳表面，然后又说在两极和赤道没有被观察到。这种说法有不科学的地方，前部分下了结论--出现在表面，后半部分只想是让步说“虽然没有在两极和赤道位置被看见”，给人一种没有证据就下结论的感觉。

A选项中前后两个部分是从可见与是否被见到的角度谈sunspots，这样逻辑上非常通顺。