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.

Sensitive

Explanation:

According to my research on studies conducted by various developmentalists, I can say that based on the information provided within the question Sergei's story illustrates a term referred to as a Sensitive Period. Based on developmentalist's research this is defined as a time when the effects of experience are particularly strong on a limited period in development

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i need more info !

the answer to your question is,

d. when a child was born.

-mabel < 3