请教一道数学题

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

答案是A。

可我怎么觉得只有一种啊。谢谢了

设从低到高分别是1，2，3，4，5，6，第一种：第一排1，2，3，第二排4，5，6 ，二种：一排1，3，4，二排2，5，6 三种：一排1，3，5，二排2，4，6，四种：1，2，4，二排3，5，6 五种：1，2，5二排3，4，6