9. Sunspots, vortices of gas associated with strong electromagnetic activity, are visible as dark spots on 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 darkspots 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
请问D答案为什么不对呢？ETS的解释说 although 必须引导一个句子，可是像although ，even though之类的转折连词不是可以跟现代分词和过去分词，省略主语吗？而且D现代完成时时态也恰当。
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