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

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