请教GWD3-33！谢谢啦

Q33:

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。不知是怎样算出来的。请指教！