4. Let the function $y=f(x)$ satisfy for all real numbers $x$
$$
f(3+x)=f(3-x),
$$

and the equation $f(x)=0$ has exactly 6 distinct real roots, then the sum of these 6 roots is
(A) 18 ;
(B) 12 ;
(C) 9 ;
(D) 0 .