【单选题】

Everyone in a particular society recognizes social roles: father, mother, child, teacher, student, police officer, store clerk, doctor, judge, political leader, and so on.Every culture expects certain types of behavior from people who play certain social roles.Anyone occupying a given position is expected to adopt a specific attitude.

A、store clerk is expected to take care of customers patiently and politely, and a judge is expected to make wise and fair decisions about laws.　　Informal social roles are not always easy to recognize, but can be identified with careful research. They are key indicators of a group’s health and happiness. Within the family, one informal role is the family hero, the person who defines integrity and upholds family morality. Others are the family arbitrator, the person who keeps the peace, and the family historian, often a grandparent, who relays valuable cultural information that maintains both the family and the larger society.And finally, there is the family friend, the person who provides comfort and companionship to the family members with emotional needs. Why does the author use the term key indicators in discussing informal social roles
A、To identify the most important type of social roles.
B．To explain how to identify informal social roles
C．To point out that informal roles are unique to families
D．To emphasize the value of informal roles to a group