In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y ?
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Axy
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Bx+y
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C\frac{1}{x+y}
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D\frac{xy}{x+y}
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E\frac{x+y}{xy}