4.[单选题]-|||-设 =f(x,y,z), 其中f具有一阶连续偏导数,而-|||-=(x)^2-2(y)^2, 则 dfrac (partial u)(partial x)= __-|||-_-|||-A dfrac (ef)(ex)-|||-B)-|||-dfrac (partial f)(partial x)+dfrac (partial f)(partial z)-|||-C dfrac (partial f)(partial x)+2xdfrac (partial f)(partial z)-|||-D dfrac (partial f)(partial x)+dfrac (partial f)(partial y)+2xdfrac (partial f)(partial z)