The Cause of the Dark Colors on the Sistine Chapel

The cause of the dark colors on the Sistine Chapel ceiling is largely the result of _______.

A) a fire in the early twentieth century that caused minimal damage to the chapel
B) the mixing of dark colors by Michelangelo himself
C) animal glues used as varnishes by past restorers
D) paint used by critics who thought the colors Michelangelo used were too bright
E) none of these are correct