Autism vs. ADHD

There was a passage in NS Psych/Soc about joint attention ability, and it had a question:
"In what disorder is an infant's impairment to jointly attend a primary symptom of the disorder?
A. Anxiety
B. Conduct Disorder
C. Autism
D. ADHD"

I picked D, but the answer was C. Their explanation is not that great; "Autism is a disorder in which social contact and language is severely disrupted."

Isn't this kind of a vague question?