What does the circular halo placed behind the head signify in the art of the Middle Ages

A．It follows a kind of mathematics.
B．It's religious art, employing symbols to convey its meanings.
C．Art becomes an allegory, beyond each painting some profound meanings are hidden.
D．Art of the Middle Ages embodies the personality of the artist in a diffused way.

A．The art of the Middle Ages is elevated by its religious and sacred facet.
B．Artists of the Middle Ages were absolutely talented.
C．The art of the Middle Ages formed its own unique system.
D．The religious works in the Middle Ages reached unparallel height in art.