5: For any function $f(x)$, in the same Cartesian coordinate system, the graphs of the functions $y_{1}=f(x-19)$ and $y_{2}=$ $f(99-x)$ are always symmetric with respect to ( ).
(A) $y$-axis
(B) line $x=19$
(C) line $x=59$
(D) line $x=99$
5. (C).
$$
\begin{array}{l}
\text { Let } x^{\prime}=x-59, y^{\prime}=y . \text { Then } \\
y_{1}^{\prime}=f\left(x^{\prime}+40\right), y_{2}^{\prime}=f\left(40-x^{\prime}\right) .
\end{array}
$$

Since the function $y=f(a+x)$ is always symmetric to the function $y=f(a-x)$ about the $y$-axis $(x=0)$,
then $y_{1}^{\prime} 、 y_{2}^{\prime}$ are symmetric about $x^{\prime}=0$.
That is, symmetric about the line $x=59$.