What have follow-up studies found about early intervention programs for infants, toddlers, and preschool children?
a) There are no lasting effects of early intervention.
b) There is a big increase in behavioral problems among children who participated in early intervention programs.
c) Early intervention programs lead to short-term benefits but no long-term effects.
d) Early intervention programs have been shown to have lasting positive effects on children's development and well-being.