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) $S_{21}$
(C)
I. Multiple Choice Question
1.【Analysis and Solution】Let the common difference of the arithmetic sequence be $d$. Then, from $3 a_{8}=5 a_{13}$, we get $3\left(a_{1}+7 d\right)=5\left(a_{1}+12 d\right)$, which simplifies to $a_{1}=-19.5 d$. Therefore, $a_{20}=a_{1}+19 d>0$ and $a_{21}=a_{1}+20 d<0$,
$\therefore$ the sum of the first 20 terms of the arithmetic sequence is the maximum, hence the answer is $(\mathrm{C})$.