Julianna Is a Nine-Year-Old Child Diagnosed with Attention Deficit Hyperactivity

Julianna is a nine-year-old child diagnosed with attention deficit hyperactivity disorder. Her mother was a chain smoker even after Julianna was conceived. Which of the following factors is chiefly responsible for her condition?

A) Julianna inherited both forms of a dopamine gene from her mother.
B) Julianna has abnormal brain growth rate.
C) Julianna consumed excess amounts of sugar as an infant.
D) Julianna has one form of a dopamine gene but not another.