$33 \cdot 69$ If an integer $n$ greater than 8 is a solution to the equation $x^{2}-a x+b=0$, and in base $n$, $a$ is represented as 18, then the base $n$ representation of $b$ is
(A) 18.
(B) 28.
(C) 80.
(D) 81.
(E) 280.
(29th American High School Mathematics Examination, 1978)
[Solution] From the given,
$$
b=a n-n^{2}=(18)_{n} n-n^{2}=(n+8) n-n^{2}=8 n=(80)_{n} \text {. }
$$

Therefore, the answer is $(C)$.