4.23 If the quadratic function $f(x)=a x^{2}+b x+c(a<0)$ satisfies $f(x+2)$ $=f(2-x)$. Then the size relationship of $f(0), f(-1992), \quad f(1992)$ is
(A) $f(0)<f(1992)<f(-1992)$.
(B) $f(1992)<f(0)<f(-1992)$.
(C) $f(-1992)<f(0)<f(1992)$.
(D) $f(-1992)<f(1992)<f(0)$.
(China Beijing High School Mathematics Competition, 1992)
［Solution］From the given, we know that the parabola opens downwards, and by $f(x+2)=f(2-x)$, i.e., $f(2+x)=$ $f(2-x)$, then $x=2$ is its axis of symmetry.
Also, $f(1990+2)=f(2-1990)$,

i.e., $f(1992)=f(-1988)$,
we know $\quad f(1992)>f(-1992)$.
$\therefore f(-1992)<f(1992)<f(0)$.
Therefore, the answer is $(D)$.