6. A simplified model of a roundabout at a three-way intersection is shown in Figure 3. During a certain peak period, the number of motor vehicles entering and exiting intersections $A$, $B$, and $C$ per unit time is as shown in the figure. In the figure, $x_{1}$, $x_{2}$, and $x_{3}$ represent the number of motor vehicles passing through segments $\overparen{A B}$, $\overparen{B C}$, and $\overparen{C A}$ per unit time, respectively (assuming that in the same segment, the number of vehicles entering and exiting per unit time is equal). The size relationship of $x_{1}$, $x_{2}$, and $x_{3}$ is ( ).
(A) $x_{1}>x_{2}>x_{3}$
(B) $x_{1}>x_{3}>x_{2}$
(C) $x_{2}>x_{3}>x_{1}$
(D) $x_{3}>x_{2}>x_{1}$
6.C.

From Figure 3, we get
$$
\begin{array}{l}
x_{3}-55+50=x_{1}, \\
x_{1}-20+30=x_{2}, \\
x_{2}-35+30=x_{3} .
\end{array}
$$

Simplifying, we get $x_{3}-5=x_{1}, x_{1}+10=x_{2}, x_{2}-5=x_{3}$. Clearly, $x_{2}>x_{3}>x_{1}$.