wp_dropdown_categories

In a multiple dropdown search I’m trying to use ‘selected’ in the array to set a default cat ID of each dropdown if no selection is made, but it keeps returning cat=0 on the search. I’ve been over and over it again assuming one of the other array values was interfering but I can’t see anything does. Each dropdown produces the children of a parent cat and I’m trying to set each default to the parent value. Anyone any clues as to why?
All In then need to do is work out how to make it inclusive!!!