A.
$\dfrac{\partial f}{\partial x }$;
B.
$\dfrac{\partial f}{\partial u }\dfrac{\partial u}{\partial x }$;
C.
$\dfrac{\partial z}{\partial u }\dfrac{\partial u}{\partial x }+\dfrac{\partial z}{\partial x }$;
D.
$\dfrac{\partial f}{\partial u }\dfrac{\partial u}{\partial x }+\dfrac{\partial f}{\partial x }$.