Why do the blues in medieval Italian frescoes often fade more than other colors?

a. They are used in areas of painting more frequently exposed to light.
b. They fade by design, as a sign of the brevity of earthly life.
c. They use minerals that don’t combine with wet plaster, which would produce more durability.
d. They are used in areas of painting more likely to be touched by worshipers.

Answer: c