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下列有关化学反应xM＋yN＝mP＋nQ的说法中，正确是 （ ）

A．化学计量数x与y的和一定等于m与n的和

B．该反应为复分解反应，且在P和Q中一定有一种物质为水

C．反应物M和N的质量比一定等于生成物P和Q的质量比

D．若M为ag、N为ag，反应后生成的P和Q的质量总和一定不是ag