站队题求解！！！

闭眼看书

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 height of the people within each row must increase from left to fight, 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 3
B 4
C 5
D 6
E 12
这种题用枚举法可以列出来，但有什么可以抽象出来的运算套路吗？比如说如果题目出的是20个人排成两排的话