[单选题]What are the definitions of the two main types of families？

A.The nuclear family consists of two parents and their biological or adopted children.
B.The extended family may include grandparents, parents, children and relatives living together on the same street or in the same area.
C.Human beings lived in loosely-related groups. Each group had a common ancestor long time ago.
D.The nuclear family is the same as the extended family.