By what evidence can art historians assert that color functions as a unifying agent for the entire decorative scheme developed by Giotto di Bondone for the Arena (Scrovegni) Chapel in Padua, Italy?
a. He used the same kind of blue from the vault for the backgrounds of the narrative panels on each wall.
b. All of Giotto's other works use color in this way, and this was a characteristic feature of his individual artist's style.
c. The contract between Giotto and the patron Enrico Scrovegni stipulated that he was to use color as his defining design characteristic.
d. The entire chapel was painted in the grisaille (monochrome gray) technique to imitate sculpted stone relief.