$1 \cdot 10$ non-zero real numbers, each triplet $(a, b, c)$ forms a number
$$
\frac{a}{|a|}+\frac{b}{|b|}+\frac{c}{|c|}+\frac{a b c}{|a b c|},
$$

The set of all such numbers is
(A) $\{0\}$.
(B) $\{-4,0,4\}$.
(C) $\{-4,-2,0,2,4\}$.
(D) $\{-4,-2,2,4\}$.
(E) None of these.
(28th American High School Mathematics Examination, 1977)
[Solution] If $a, b$ and $c$ are all positive or all negative, then the expression forms 4 or -4, respectively. In other cases, the number formed by the expression is 0.
Therefore, the answer is $(B)$.