求助，谁能解释一下？

何小媛159753

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A，C为什么不对呢？
这句话讲的是什么意思？

何小媛159753

谢谢版主

毛毛carina

http://forum.chasedream.com/GMAT_SC/thread-406669-1-1.html

何小媛159753

OG里解释C时说，Although要补齐句子，但有时候它后面也省略主谓，所以不太清楚什么时候能省什么时候不能省？

毛毛carina

大概意思是，sunspots 像太阳表面的duckspots一样能够被看见，但是在赤道和两极从未被看见过

毛毛carina

这道题是OG11的题目噢，我给你复制一下解释
The correct parallel structure in the original sentence emphasizes the contrast between where sunspots are found (are visible...Sun) and where they are not (have ever beensighted...equator). Sunsports is the subject of the sentence, are is the verb of the first part of the contrast and have been sighted is the verb of the the second. The adjective visible is a complement and is a parallel to the past participle sighted. Both parts of the sentence conclude with phrases indicating location.The contrast itself is indicated by the conjunction but.
A.correct. This sentence clearly and correctly draws a contrast between where sunspots are found and where they are not
B.Changing the modifying clause so that that never...Sun distorts the meaning of the sentence; the contrast is lost
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
E.The relative pronoun which should immediately follow its referent; here which illogically follows surface, and its intended referent, either sunspots or dark spots, becomes unclear

何小媛159753

那个sunspots 和 dark spots是一个东西吗？