5.设 =dfrac (y)(x)f(xy), 其中函数f可微,则 dfrac (x)(partial )dfrac (partial z)(partial x)+dfrac (partial z)(partial y)= () .-|||-A.2yf`(xy) B. -2yf'(xy) C. dfrac (2)(x)f(xy) D. -dfrac (2)(x)f(xy)