What Is the independent Variable in the Following Statement

What is the independent variable in the following statement?
"A third hypothesis for family effects states that the continuing absence of a father leads to low educational attainment among offspring in single-parent families. This view is derived from socialization theory, which stresses the importance of the male role model to the cognitive and emotional development of children."
American Journal of Sociology (1985, p. 889)

A) educational attainment
B) single-parent family
C) socialization theory
D) absence of father in the family
E) emotional development of children