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17. Sunspots, vortices of gas associated with strong electromagetic 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. 如原题
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
正确选A
请问but后面的have never been sighted on是和前面的are时态不一样啊。不需要平行吗这里?
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