Suan did not learn to read until adolescence. When they did learn to read, it was much harder for them than it was for their peers. If Suan had learned to read during the childhood years, they would have acquired literacy skills much more competently. Suan's difficulty in learning to read as an adolescent demonstrates the concept of a(n) "_____ period." 

a. early 

b. late 

c. critical 

d. sensitive