I need a query which will select from a table a distinct row respresenting the rows which have the least number of occurrences...

e.g:

A 1
A 2
B 1
B 2
B 3
C 1
C 2
C 3
C 4
D 1
D 2
E 1
E 2
E 3
F 1
F 2


From the above data set I would get back:
A
D
F

as each of these letters appears the least number of times (2) in the table.

I am stuck on this one...