The best title of the passage might be ______.

A．the conflicts between the peasants and the landed aristocrats
B．the foreign residents
C．slaves
D．the newly emerging middle class in the sixth century

A．to define the term social class does not involve much difficulty
B．there is much alternation in people's social classes
C．to evaluate a person's social class is a very complex procedure
D．we can tell which social class a person belongs to by the way he behaves