$29 \cdot 50$ If $x^{2}+3 x y+x+m y-m$ has two linear factors with integer coefficients in $x, y$, then the set of values of $m$ is
(A) $\{0,12,-12\}$.
(B) $\{0,12\}$.
(C) $\{12,-12\}$.
(D) $\{12\}$.
(E) $\{0\}$.
(15th American High School Mathematics Examination, 1964)