There is a feature request for LOV's to be able to reference a value in the item itself. There is an option to achieve this in a FORM but not a TABULAR FORM. It works like this on a form, for instance:
select thing_id r, thing_name d from table_of_things where thing_type = 'FILTER' and ( active_flag = 'Y' or thing_id = :P99_THIS_FIELD )
This way you can filter out the value already in the item (B), but your logic behind not showing C is unclear to me.
And vote for feature request AC3E (and to see more background info on this type of LOV).