A mother brings in her 9-year-old son, Brock, for evaluation. She has received several calls from his teacher regarding disruptive behavior in school, including not following directions and being argumentative to teachers. At home, he is easily annoyed and quick to lose his temper. Further assessment reveals that Brock will focus and complete his schoolwork when pressed, gets good grades, and plays well with other children. What would be the most likely diagnosis ?Expalin