GWD数学的一道题 请大牛帮忙

aolynne

GWD第一套中    Q25Aphotographer will arrange 6 people of 6 different heights for photograph byplacing them in two rows of three so that each person in the first row isstanding in front of someone in the second row. The heights of the people within each row must increase from left toright, and each person in the second row must be taller than the personstanding in front of him or her.  Howmany such arrangements of the 6 people are possible?
A.   5

B.   6
C.   9
D.   24
E.   36
请问怎么 做？？ 数学不好，尤其是排列组合。。。。请大家辅导一下！

lhz910216

我直接列出了5种具体情况看的，找不到第6种了，就选A了