1. Given $\left\{a_{n}\right\}$ is an arithmetic sequence, $S_{n}$ is the sum of its first $n$ terms. Then $-a_{m}<0$, $S_{m+1}<0$ is a ( ) condition.
(A) Sufficient and necessary
(B) Sufficient but not necessary
(C) Necessary but not sufficient
(D) Neither sufficient nor necessary
-,1. A.
In fact,
$$
\begin{array}{l}
\left\{\begin{array}{l}
S_{m}=\frac{m}{2}\left(a_{1}+a_{m}\right)>0, \\
S_{m+1}=\frac{m+1}{2}\left(a_{1}+a_{m+1}\right)<0
\end{array}\right. \\
\Leftrightarrow a_{1}+a_{m+1}<0<a_{1}+a_{m}
\end{array} \Leftrightarrow
$$

Therefore, it is a sufficient and necessary condition.