25-12 If the graphs of the equations $3 x+b y+c=0$ and $c x-2 y+12=0$ are the same, let $n$ be the number of pairs of values of $b, c$. Then $n$ is
(A) 0.
(B) 1.
(C) 2.
(D) Finite but more than 2.
(E) Greater than any finite number.
(15th American High School Mathematics Examination, 1964)
[Solution] From the given, we have $3: c=b:-2=c: 12$, let it be $k$ then we have
$$
3=c k, \quad b=-2 k, \quad c=12 k .
$$

Solving this yields $c=6, b=-1$ or $c=-6, b=1$.
Therefore, the answer is $(C)$.