Query about Question 4 from "(I'm Overwhelmed) Arithmetic Progress Quiz #5" [6955]

[user12014]

https://www.gregmat.com/quizzes/quiz/im-overwhelmed-arithmetic-progress-quiz-5 (Question 4)

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Why is the answer not 4? I’m confused why don’t count 1 as a distinct prime factor.

[Kweku]

1 is not a prime number