An Association Has Been Found Between Postpartum Depression and Infant

An association has been found between postpartum depression and infant characteristics.What is a major difficulty involved in interpreting this relationship?

A) There has been no genotype discovered that is directly linked with postpartum depression.
B) It cannot be determined if the postpartum depression caused poor infant characteristics or if the poor infant characteristics caused postpartum depression.
C) There has been no genotype discovered that is directly linked with poor infant characteristics.
D) The mother's and infant's genotype are too closely related to interpret the relationship.