I've read in JQuery docs that data can be an array (key value pairs). I get the error if I put:
This is object not an array:
var data = {
'mode': 'filter_city',
'id_A': e[e.selectedIndex]
};
You probably want:
var data = [{
'mode': 'filter_city',
'id_A': e[e.selectedIndex]
}];