Case 2: The three close pairs form a path: \( AB, BC, CD \). Edges: AB, BC, CD. Trio \( A,B,C \): AB, BC close → 2 close pairs. Trio \( B,C,D \): BC, CD close → 2 close pairs. Trio \( A,B,D \): AB, BD? BD not close, AD not close → only AB → 1 close → fails. Trio \( A,C,D \): AC not close → 0 → fails. So only the two middle trios (A,B,C) and (B,C,D) each have exactly 2 close pairs. - Portal da Acústica