Research Suggests the Reason Why First-Born Children Have Higher IQs

Research suggests the reason why first-born children have higher IQs than their siblings is that first-born children:

A) tend to be favored by their parents throughout childhood.
B) have exclusive access to their parents' child-rearing until siblings are born.
C) on average, are exposed to fewer toxins in the womb.
D) are more likely to be breastfed than their siblings.