Query about Question 8 from "(I'm Overwhelmed) Data Analysis Progress Quiz #7" [8711] - confusion bw option 7 and 8

[user6909]

https://www.gregmat.com/quizzes/quiz/im-overwhelmed-data-analysis-progress-quiz-7 (Question 8) -

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Why did Greg exclude the empty set for the solution to this ques?

[Kweku]

Refer to this please.