the 6th

antonia

P57 262
   Sunspots, vortices of gas associated with strong electro-magnetic activity, [U]are visible as dark spots on the surface of the Sun but have never been sighted on [/U]the Sun’s poles or equator.
   (A) are visible as dark spots on the surface of the Sun but have never been sighted on
   (C) appear on the surface of the Sun as dark spots although never sighted at
尽管我承认A并不好，可我觉得作为正确答案的C好像错误更多，dark spots 是否应紧跟appear? 还有have never been时态有错吗？

linsonstudy

appear当然可以直接接在dark spots后了。SC并不是选出正确的句子，而是选出最好的句子。
C 的表达既正确又简洁。