What is a function of the mirror in Diego Velázquez’s The Maids of Honor (Las Meninas) and Jan van Eyck’s The Arnolfini Portrait?

a)  Both reveal a portrait within a portrait.
b)  Both act as the vanishing point in the painting.
c)  Both present significant religious content.
d)  Both contain the artist’s signature.

a)  Both reveal a portrait within a portrait.