In a saucepan stir sugar into 2 cups water. Bring to a boil, stirring until sugar dissolves. Using a sharp knife, cut peel away from oranges, lemons and lime, taking care not to include bitter white pith. Add peels to syrup. Squeeze fruits; add juice to syrup. Let cool, cover and steep for 8 hours. Strain mixture into a pitcher. Add ice to 8 glasses. Fill glasses halfway with syrup and top with chilled seltzer.
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