请问这个怎么解呢？

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

我用的公式

6个人，两个人一组，所以两种组合。。。

6X5 / (2X1) = 15 所以答案在15以下。

其中一般都不可以，应为一半是高的在前面对推理。

怎么系统化知道是5呢？