mb 여분필드 select박스로 입력하려고해요..도와주세요..

<div class="form-group has-feedback text-gap">
			<label class="col-sm-2 control-label" for="reg_mb_1"><b>분야</b><strong class="sound_only">필수</strong></label>
			<div class="col-sm-3">
                <select name="mb_1" id="reg_mb_1" required class="form-control input-sm nospace" onchange="itemChange()">
                    <option value="">선택하세요</option>
					<option value="A" <?php echo get_selected($member['mb_1'],"A");?>>A</option>
					<option value="B" <?php echo get_selected($member['mb_1'],"B");?>>B</option>
                </select>
			</div>
			</div>
			
			<div class="form-group has-feedback text-gap">
			<label class="col-sm-2 control-label" for="reg_mb_2"><b>2차 분야</b><strong class="sound_only">필수</strong></label>
			<div class="col-sm-3">
                <select name="mb_2" id="reg_mb_2" class="form-control input-sm nospace">
				 <option value="">선택하세요</option>
                </select>
			</div>
			</div>
<script>
function itemChange(){
 
var abc = ["1","2","3"];
var def = ["4","5","6"];
 
 
var selectItem = $("#reg_mb_1").val();
 
var changeItem;
  
if(selectItem == "A"){
  changeItem = abc;
}
else if(selectItem == "B"){
  changeItem = def;
}
 
$('#reg_mb_2').empty();
 
for(var count = 0; count < changeItem.length; count++){                   
                var option = $("<option>"+changeItem[count]+"</option>");
                $('#reg_mb_2').append(option);
            }
 
}
 
</script>