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 the first $n$ terms, then the $n$ that makes $S_{n}$ the largest is $(\quad)$
A. 18
B. 19
C. 20
D. 21
1. $\mathrm{C}$ Let the common difference be $d$, then we have $3\left(a_{1}+7 d\right)=5\left(a_{1}+12 d\right)$, which gives $a_{1}=-\frac{39}{2} d$, and from $a_{1}>0$ we know $d<0$, $a_{21}=a_{1}+20 d=\frac{1}{2} d<0$, thus $S_{20}$ is the largest.