Adverbial clause is an adverb which is a dependent clause. It's used as an adverb in a sentence to show the time, contrast, conditions, place, reason, concession, purpose or result.
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Since this is during the period of the Soviet Union and communism, Kennedy is asking the people to come together and defend freedom.
'Memoir' is the correct answer, because in it, the author talks about his own life and personal experiences.
