As a young boy, Sergei lived in a country that endured years of civil unrest. As a result, he was unable to attend elementary school on a regular basis, leading to deficits in reading and writing. As an adult, Sergei enrolled in adult learning classes to improve his reading and writing skills. Sergei's story illustrates what developmentalists refer to as __________ periods.
a. Sensitive
b. Irreversible
c. Critical
d. Dysfunctional