13. (Sichuan Province) Let the function $f(x)$ satisfy $f(1+x)=f(1-x)$ for all real numbers. If the equation $f(x)=0$ has exactly two distinct real roots, then the absolute value of the difference between these two roots is ( ).
(A) 1.
(B) 2.
(C) 4.
(D) Uncertain.
D

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