A 7-year-old boy comes to the office because has been failing to do schoolwork and his mother is concerned about his ability to focus. He has difficulty paying attention in school, staying quietly seated during class, and often interrupts without waiting for his turn to speak. Which of the following criteria must be present in order to diagnose attention deficit hyperactivity disorder?

a. Overwhelming fear and worry in response to schoolwork
b. Signs of impulsivity at school only
c. Evidence of social skill impairment
d. Symptoms present for at least 6 months