Which of the Following Pieces of Evidence Is the Best

Which of the following pieces of evidence is the best support for the causal effect of infant attachment security on later social functioning?

A) Children who were securely attached as infants are more likely than other children to be socially and emotionally competent.
B) When security of attachment changes because of stress in the home, parent-child interactions at the time are associated with social and emotional competence at that age.
C) Among children who functioned poorly during preschool, those who had a secure attachment during infancy were more socially and emotionally competent in middle childhood than were other children.
D) None of these answers supports this effect.