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Relation R(a,b) has no duplicate tuples, although neither a nor b is a key. Relation S(c,d) may have duplicates. Which of the following queries is guaranteed to produce no duplicates?

		a)	SELECT b FROM R WHERE a IN (SELECT d FROM S)
		b)	SELECT c, d FROM S WHERE EXISTS (SELECT a, b FROM R WHERE a = c AND b = d)
		c)	SELECT c, d FROM S GROUP BY c, d
		d)	SELECT b FROM R