请教一道GWD数学题, 没有解题思路,还请高人指教!

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?
A5
B6
C9
D24
E36

Denote the six people by 1, 2, 3, 4, 5, and 6, in order of their heights and with 1 the shortest and 6 the tallest.

1 must be the leftmost of the first row.  6 must be the rightmost of the second row.  Let us now place the 2.  There are only two positions for him:
_ _ 6
1 2 _
or
2 _ 6
1 _ _

In the first case, there are 3 possibilities:
4 5 6
1 2 3
or
3 5 6
1 2 4
or
3 4 6
1 2 5

In the second case, there are 2 possibilities:
2 4 6
1 3 5
or
2 5 6
1 3 4

Thus the total number of possibilities is 5.

thanks!