设函数f ( x )可导,y= f ( x )cos f ( x )的导数为（ ）.
A:y'= f′( x )cos f ( x )- f( x )sin (f ( x )) f′( x )
B:y ′=-f′( x )sin f ( x )
C:y ′= f′( x )cos f ( x )+ f( x )sin (f ( x )) f′( x )
D:y ′= f′( x )cos f ( x )-f( x )sin (f ( x ))