[求助] 请教 CXD Page37 Q28 的解题思路

CXD  Page37, Q28: 

                       Ave A        Ave B       Ave C

                         |               |              |

4th Street ---------+--------------+--------------Y

                         |               |              |

3rd Street ---------+--------------+--------------+

                         |               |              |

2nd Street --------+--------------+--------------+

                         |               |              |

1rt Street -------- X  -------------+-------------+

                         |               |              |

Pat will walk from intersection X to intersection Y along a route that is confined to the square grid of four streets and three avenues shown in the map above. How many routes from X to Y can Pat take that have the minimum possible length?

(A) Six  (B) Eight   (C) Ten   (D) Fourteen  (E) Sixteen

     看了CXD的题解，还是不明白 2, 3, 4, B, C 排列是如何得来的，请各位NN指点！ 先谢了！            

cxd的解释挺清楚的了吧,就是2,3,4和B,c的顺序是已经确定的了，必须是2-3-4,B-C,现在就是把B,C往2-3-4中插,且B必须在C的前面,则有C5^2=10种排法