2. Let $S_{n}$ be the sum of the first $n$ terms of the arithmetic sequence $\left\{a_{n}\right\}$. If $S_{5}=S_{9}$, then $a_{3}: a_{5}=(\quad)$
(A) $9: 5$
(B) $5: 9$
(C) $3: 5$
(D) $5: 3$
2. A.
$$
S_{5}=5 a_{3}, S_{9}=9 a_{5} \text {. }
$$

From $5 a_{3}=9 a_{5}$, we get $a_{3}: a_{5}=9: 5$.