需要将select标记选项值传递给ajax函数

当我按下添加按钮时，我需要获得选择标签值的值

这是我的ajax文件

function insertDep(){
   var ajaxRequest;
   try{
      ajaxRequest = new XMLHttpRequest();
   }catch (e){
      try{
         ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
      }catch (e) {
         try{
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
         }catch (e){
            alert("Your browser broke!");
            return false;
         }
      }
   }
   ajaxRequest.onreadystatechange = function(){   
      if(ajaxRequest.readyState == 4){
         var ajaxDisplay = document.getElementById('addmegp');
         ajaxDisplay.innerHTML = ajaxRequest.responseText;
      }
   }
   var unitnamedept = document.getElementById('unitnamedept').value;

   var queryString =  "?unitnamedept=" + unitnamedept;
   ajaxRequest.open("GET", "insertDept.php" + queryString, true);
   ajaxRequest.send(null); 
}

这是我的html文件

<select> <option name="unitnamedept" id="unitnamedept" value="0" >--</option> <option value="D01">D01</option> <option value="D02">D02</option> <option value="D03">D03</option> <option value="D04">D04</option> </select>
<input type="button" name="ang" align="right" value="Add" onclick="insertDep()"/>

**我有另一个名为insertDept.php的页面**我使用此页面显示

<?php
$unitnamedept = $_GET['unitnamedept'];
echo "$unitnamedept";
?>

当我按下只收到0值的按钮时，请提供任何帮助