1. (1999 Henan High School Mathematics Competition) Let $S_{n}$ be the sum of the first $n$ terms of the arithmetic sequence $\left\{a_{n}\right\}$. $S_{9}=18, a_{n-4}=$ $30(n>9), S_{n}=336$, then the value of $n$ is
A. 16
B. 21
C. 9
D. 8
1. Choose B.

Slightly explained: Since $S_{9}=\frac{9\left(a_{1}+a_{9}\right)}{2}=\frac{9\left(a_{5}+a_{5}\right)}{2}$, we have $a_{5}=2$. And $S_{n}=\frac{n\left(a_{1}+a_{n}\right)}{2}=\frac{n\left(a_{5}+a_{n-1}\right)}{2}=\frac{n(2+30)}{2}=336$, so $n=21$.