求助：GWD31-1-Q25 数学部分：哪位高手指点一下思路？

zhangkou

这道题我自己用弱智方法（一个一个排）算出来了答案， 拜求各位高手指点简捷思路！！！！！
Q25:
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
Answer: A