select

I have a table with fields:
id      A       B       C       D       E       F
where id is an int4 primary key.
In this table there is information like:
1       a1      b1      xxx             xxx                     
2       a1      b1      xxx     xxx     xxx     xxx
3       a2      b2      xxx     xxx     xxx     xxx
4       a2      b2      xxx     xxx     xxx     xxx
5       a3      b3      xxx             xxx     xxx
6       a3      b3      xxx     xxx     xxx     xxx
7       a3      b3      xxx     xxx     xxx     xxx
8       a4      b4      xxx     xxx     xxx     xxx
9       a1      b1      xxx
10      a3      b3      xxx
11      a1      b3              xxx
where xxx represents any information. 
My problem is: I only want to select one of the records which have the same
combination of information in fields A and B, and that should be the record
with the lowest id. Thus, the resultset should be:
1       a1      b1      xxx             xxx                     
3       a2      b2      xxx     xxx     xxx     xxx
5       a3      b3      xxx             xxx     xxx
8       a4      b4      xxx     xxx     xxx     xxx
11      a1      b3              xxx
Importantly, I might not want the selected records listed in order of increasing
id. They might be sorted according to the data in e.g. the C field.

Suggestions/comments greatly appreciated.