Imo A
Lets analyse each answer choice
(B) are visible as dark spots that never have been sighted on the surface of the Sun
This choice changes meaning and leaves some part of the original sentence
(C) appear on the surface of the Sun as dark spots although never sighted at
C seems to suggest that it is a universal truth by using although never sighted at .
But it is case that they have not been sighted at sun's poles or equator , it can in future .That is what the sentence is trying to convey
...
Lets analyse each answer choice
(B) are visible as dark spots that never have been sighted on the surface of the Sun
This choice changes meaning and leaves some part of the original sentence
(C) appear on the surface of the Sun as dark spots although never sighted at
C seems to suggest that it is a universal truth by using although never sighted at .
But it is case that they have not been sighted at sun's poles or equator , it can in future .That is what the sentence is trying to convey
...
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