In Changing from Cartesian to Polar Coordinates $\frac { \partial r } { \partial x }$

Multiple Choice

In changing from Cartesian to polar coordinates, $\frac { \partial r } { \partial x }$ is

A) $\sin \theta$
B) $\cos \theta$
C) $\sin \theta / r$
D) $\cos \theta / r$
E) $- \sin \theta / r$