$29 \cdot 47$ Polynomial $2 x^{4}-3 x^{3}+a x^{2}+7 x+b$ can be divided by $x^{2}+x-2$, then the value of $\frac{a}{b}$ is
(A) -2 .
(B) -12 .
(C) 6 .
(D) 4 .
(E) 3 .
(China Beijing Junior High School Mathematics Competition, 1984)
［Solution］Let $2 x^{4}-3 x^{3}+a x^{2}+7 x+b$
$$
\equiv\left(x^{2}+x-2\right)\left(2 x^{2}+m x+n\right)
$$

By comparing coefficients, we get $\quad a=-12, \quad b=6$.
Therefore,
$$
\frac{a}{b}=-2 \text {. }
$$

So the answer is $(A)$.