设f(u)具有二阶连续导数，且(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)),则(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)), A．(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)), B．(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)), C．(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)), D．(x,y)=f(dfrac (y)(x))+yf(dfrac (x)(y)),

设f(u)具有二阶连续导数，且则
    A．
    B．
    C．
    D．