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
