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Let a be a nonempty subset of an integer set. For K ∈ a, if k-1 ∉ A and K + 1 ∉ a, then K is said to be a "good element" of A. given s = {1, 2, 3, 4, 5, 6, 7, 8}, all the sets composed of three elements of s have () A. 6 B. 12 C. 9 D. 5
If there is no "good element", it means that the three numbers must be connected together (if they are not connected together, the separated number is "good element"). Therefore, the set without "good element" has six possibilities {1, 2, 3}, {2, 3, 4}, {3, 4, 5}, {4, 5, 6}, {5, 6, 7}, {6, 7, 8}, so a is selected
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