Find the Differential $d y \text { of the function } y = x \cos ( 7 x ) \text {. }$

Multiple Choice

Find the differential $d y \text { of the function } y = x \cos ( 7 x ) \text {. }$

A) $7 x \sin ( 7 x ) d x$
B) $\cos ( 7 x ) d x$
C) $\cos ( 7 x ) + 7 x \sin ( 7 x ) d x$
D) $\cos ( 7 x ) - 7 x \sin ( 7 x ) d x$
E) $- 7 x \sin ( 7 x ) d x$