OG11-9,OG11-15 关于逗号

OG11-9

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

ANS:A

答案是没有什么问题，但是有点对于A还是有点不明白，关于but，是不是象A那样，在用but连接2个句子，并且由于前后2个句子主语相同省略后面句子主语时，but前面必须去掉comma.

如果comma必须去掉的话，象OG11-15这样的题（如下）

15. The Iroquois were primarily planters, but supple­mentinq their cultivation of maize, squash, and beans with fishing and hunting.

同意你的看法，在省略的情况下，逗号应该去掉（只要前面没有插入语）