单项选择题设函数z=f(x,y)=xy/(x2+y2),则下列结论中不正确的是()

A.f(1,y/x)=xy/(x2+y2
B.f(1,x/y)=xy/(x2+y2
C.f(1/x,1/y)=xy/(x2+y2
D.f(x+y,x-y)=xy/(x2+y2


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