justify the study of Nigeria people and culture for having positive impact in uniting Nigeria's ethnic group

Study of the Nigerian people and its vast spread of ethnic groups can help in unifying the diverse ethnic groups as one understand the culture conventions and principles associated to each ethnic group making them to understand one another which can promote unity.

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d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."

answer; severe;