A sibling fixed effects model shows the Head Start pre-school program improves children's high school graduation rates. Which of these is a valid argument that this might be an underestimate of the program's impacts on the population studied?
A. Families who use Head Start care more about education than families who do not.
B. Families who use Head Start are lower income than families who do not.
C. Parents are more likely to use Head Start for a child with academic challenges.
D. Parents are more likely to use Head Start for a child with academic strengths.
Answer: C. Parents are more likely to use Head Start for a child with academic challenges.