15. (12 points) A small beetle starts from point $A$ and walks along line segments or arcs to point $B$, without repeating any point along the way. How many different routes can the beetle take?
A. 64
B. 72
C. 128
D. 144
【Answer】Solution: According to the analysis, 
there are: $4 \times 4 \times 4 \times 2=128$ (ways)
Answer: The beetle can take a total of 128 different routes.