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.
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Aare visible as dark spots on the surface of the Sun but have never been sighted on
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Bare visible as dark spots that never have been sighted on the surface of
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Cappear on the surface of the Sun as dark spots although never sighted at
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Dappear as dark spots on the surface of the Sun, although never having been sighted at
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Eappear as dark spots on the Sun's surface, which have never been sighted on
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正确答案: A