6. The sequence $\left\{a_{n}\right\}$ has the sum of its first $n$ terms as $S_{n}$. If $S_{n}=3+2 a_{n}\left(n \in \mathbf{N}_{+}\right)$, then this sequence must be ( ).
A. A geometric sequence
B. An arithmetic sequence
C. $a_{n}(n \geqslant 2)$ is an arithmetic sequence
D. $a_{n}(n \geqslant 2)$ is a geometric sequence
6. A. From $S_{n}=3+2 a_{n}$, and $S_{n-1}=3+2 a_{n-1}$, thus $a_{n}=2 a_{n-1}(n \geqslant 2)$.