Which of the following equations is true for all positive values of x and y?
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A$$\sqrt{x}+\sqrt{y}=\sqrt{x+y}$$
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B$$\sqrt{x^{4}y^{16}}=x^{2}y^{4}$$
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C$$(x\sqrt{y})(y\sqrt{x})=x^{2}y^{2}$$
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D$$y\sqrt{x}+y\sqrt{x}=\sqrt{4xy^{2}}$$
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E$$(x^{y})(y^{y})=({xy})^{2y}$$
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正确答案: D