13.8 Let $x$ be a real number, $y=|x-1|+|x+1|$. Among the following four conclusions:
( I ) $y$ has no minimum value.
( II ) There is only one $x$ that makes $y$ reach its minimum value.
(III) There are a finite number of $x$ (more than one) that make $y$ reach its minimum value.
(IV) There are infinitely many $x$ that make $y$ reach its minimum value.

Which of the following is correct?
(A) I .
(B) II .
(C) III .
(D) $\mathbb{N}$.
(China Junior High School Mathematics League, 1993)
［Solution］Notice that the given function is equivalent to:
$$
y=\left\{\begin{array}{ll}
2, & -1<x<1 ; \\
-2 x, & x \leqslant-1, \\
2 x, & x \geqslant 1 .
\end{array}\right.
$$

Obviously, 2 is the minimum value of the above function. Also, from the above expression, we know that there are infinitely many $x$ values that make $y$ take the minimum value of 2.
Therefore, the correct choice is $(D)$.