Developmental Psychologists Have Attempted to Identify the Effects of Many

Developmental psychologists have attempted to identify the effects of many specific child-rearing practices on children's personality traits. The problem with this line of research is that:

A) parents who use physical punishment won't be honest with the researchers.
B) psychologists cannot agree on whether to use cross-sectional or longitudinal approaches.
C) parents are inconsistent from day to day and over the years.
D) it is difficult to recruit parents to serve as participants in this research.