6. Let the sequence $\left\{a_{n}\right\}$ satisfy $a_{1}=3$, and $a_{n+1}=a_{n}^{2}-$ $(3 n-1) a_{n}+3$. Then the sum of the first 11 terms of the sequence $\left\{a_{n}\right\}$, $S_{11}=(\quad)$.
(A) 198
(B) 55
(C) 204
(D) 188
6. A.

Substituting $a_{1}=3$ yields $a_{2}=6$, substituting $a_{n}=3 n$ yields $a_{n+1}=3(n+1)$.