Selection from ListBox

I have two listBoxes list two fields of a table. the relationship between first field and 2nd field is 1-M. i.e.
field1 field2
A       1
A       2
B       1
B       2
then I have a GridView to bind this table.
I want to select one item from listBox1 and then field2 only list the items their value are A. How to do this in C#?
Thanks.