To Study the Effects of Genes on Infants' Stress Response

To study the effects of genes on infants' stress response, researchers can

A) Compare monozygotic and dizygotic twins when they are separated from their mother
B) Compare adopted and non-adopted infants when they meet a stranger
C) Compare siblings when they fight with each other
D) Compare infants' interactions when their mothers work full-time or when their mothers stay at home with them