6. From the sequence $\left\{a_{n}\right\}$ satisfying
$$
a_{1}=a_{2}=1, a_{n+2}=a_{n+1}+a_{n}(n \geqslant 1)
$$

the terms that are divisible by 3 are extracted to form the sequence $\left\{b_{n}\right\}$. Then $b_{100}=(\quad)$.
(A) $a_{100}$
(B) $a_{200}$
(C) $a_{300}$
(D) $a_{400}$
6. D.

It is easy to know that $a_{4 k}(k \geqslant 1)$ is divisible by 3, hence the answer is D.