Which of the following is NOT a piece of evidence described in the text indicating that sleep problems cause ADHD symptoms?
a. Children with ADHD exhibit more restless, low-quality sleep.
b. Restricting children's sleep, even by 1 to 2 hours per night, makes inattention and hyperactive-impulsive symptoms worse.
c. Sleep problems at 2- to 4 years of age predict the emergence of ADHD symptoms at age 5.
d. Children with persistent sleep problems show more ADHD symptoms in later childhood than children whose early sleep problems subsided.