$140 A B C D-A_{1} B_{1} C_{1} D_{1}$ is a rectangular prism, $A$ and $C_{1}$ are two vertices that are centrally symmetric. A moving point starts from $A$ and travels along the edges of the rectangular prism to the point $C_{1}$, and each vertex is visited at most once (not all edges need to be traversed). The number of different possible routes is
A. 9
B. 15
C. 18
D. 6
140 C. Starting from $A$, there are 3 edges to choose from. Let's assume we move along $A B$. Upon reaching $B$, there are 2 edges to choose from, let's assume we move along $B B_{1}$. Then, there are 3 possible routes, namely $B_{1} C, B_{1} A_{1} D_{1} C_{1}$, $B_{1} A_{1} D_{1} D C C_{1}$. Therefore, there are a total of $3 \times 2 \times 3=18$ routes.