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.