As it is presented in the passage, rather than a smooth progression the development of the human character and personality is a series of______

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

A．The Criteria for Classifying Social Groups
B．Social Classes
C．Changes in Social System
D．The Evolution of Human Society