6. Let $\left\{a_{n}\right\}$ and $\left\{b_{n}\right\}$ be an arithmetic sequence and a geometric sequence, respectively, and $a_{1}=b_{1}=4, a_{4}=b_{4}=1$. Then which of the following conclusions is correct? ( ).
(A) $a_{2}>b_{2}$
(B) $a_{3}b_{5}$
(D) $a_{6}>b_{6}$
6. A.

Let the common difference of the arithmetic sequence be $d$, and the common ratio of the geometric sequence be $q$.
Given $a_{1}=b_{1}=4, a_{4}=b_{4}=1$, we get
$d=-1, q=\frac{\sqrt[3]{2}}{2}$.
Thus, $a_{2}=3, b_{2}=2 \sqrt[3]{2} ; a_{3}=2, b_{3}=\sqrt[3]{4}$;
$a_{5}=0, b_{5}=\frac{\sqrt[3]{2}}{2} ; a_{6}=-1, b_{6}=\frac{\sqrt[3]{4}}{4}$.