6. For $a>3$, let
$$
\begin{array}{l}
x=3 \sqrt{a-1}+\sqrt{a-3}, \\
y=\sqrt{a}+3 \sqrt{a-2} .
\end{array}
$$

Then the relationship between $x$ and $y$ is ().
(A) $x>y$
(B) $x<y$
(C) $x=y$
(D) cannot be determined