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

A．the average man often uses in his own vocabulary what was once technical language not meant for him
B．various professions and occupations often interchange their dialects and jargons
C．there is always a clear-cut nontechnical word that may be substituted for the technical word
D．an educated person would be expected to know most technical terms

A．a linguist
B．an attorney
C．a scientist
D．an essayist