1. Let the arithmetic sequence $\left\{a_{n}\right\}$ satisfy $3 a_{8}=5 a_{13}$ and $a_{1}>0, S_{n}$ be the sum of its first $n$ terms. Then the largest $S_{n}(n \in \mathbf{N})$ is
A. $S_{10}$
B. $S_{11}$
C. $S_{20}$
D. $S_{21}$
Solve $5 a_{13}-3 a_{8}=a_{1}+a_{40}=a_{20}+a_{21}=0 \Rightarrow\left\{\begin{array}{l}a_{20}>0, \\ a_{21}<0\end{array} \Rightarrow S_{20}\right.$ is maximum. So the answer is $C$.