There are X girls in a class, and their average height is M inches. In the same class, there are Y boys with an average height of N inches. What is the average height of all the students in the class?
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A$${X+Y}\over{M+N}$$
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B$${M+N}\over{X+Y}$$
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C$${XM+YN}\over{M+N}$$
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D$${XM+YN}\over{X+Y}$$
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E$$\frac{MN}{XY}$$
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正确答案: D