The Authors of the Textbook Believe That the Principal Reason

The authors of the textbook believe that the principal reason for the slow pace of deep history was:

A) Because collecting vegetal foods, fishing, and hunting are not safe or dependable ways of securing nourishment.
B) The fact that foragers had more leisure time than farmers.
C) The conscious effort of foragers to limit population growth and group size.
D) Because all humans retreated southward during the last Ice Age.