A photographer will arrange 6 people of 6 different heights for photograph by placing them in two rows of three so that each person in the first row is standing in front of someone in the second row.  The heights of the people within each row must increase from left to right, and each person in the second row must be taller than the person standing in front of him or her.  How many such arrangements of the 6 people are possible?

A.    5

B.    6

C.    9

D.    24

E.    36

好像没有太好得方法，但可以先通过逻辑推断确定最高最低两个人得位置，设6个人高度为1，2，3，4，5，6，位置为a,b,c,d,e,f待定：

上排从左到右：a,b,c

下排从左到右：d,e,f

因为d<a<b<c,d<e<f,所以d最小，d=1

右因为c>b>a>d, c>f>e, 得到c最大，c=6

这样可以确定得排列顺序为：

a,b,6

1,e,f

剩下4个数字：2，3，4，5

又因为b>a,b>e, 所以至少有两个数比b小，b只能选4，5。

又因为f>e,所以至少有一个数比f小，f只能选3，4，5

这样剩下得2就只能放在a,e得位置上，接下来就只好一个个试了。

1.a=2,b=4=>e=3,f=5

2.a=2,b=5=>e=3,f=4

3.e=2,b=4=>a=3,f=5

4.e=2,b=5=>a=3,f=4或者a=4,f=3

所以一共5种