$1 \cdot 10$ 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)