Hi
e-gmat,
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
OG13 in rejecting (C) gives below reasoning:
Although typically introduces a subordinate
clause, which has a subject and a verb,but
here there is no subject and sighted'is not a
complete verb.