Developmental milestones are helpful in determining if an infant has a problem that needs attention. Which child does not seem to be reaching language milestones at the expected age?

a) At 2 months of age, Don starts cooing.
b) Diane begins babbling at 6 months of age.
c) Carlos said his first clearly understandable words at 18 months.
d) At the age of 18 months, Carol has a vocabulary of approximately 50 words.

C