jquery change drop down value but on code behind selected value same every time

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C#3.0

C#

ASP.NET

hi
i have added options in drop down list through jquery on runtime.
on code behind i want to access selected value of drop down list that is
generated through jquery .

but when i check selected value of drop down list at button click.
it show always first list item value as selected value.

My code on aspx page

function fillmodels(){
var makeid=$("select[id$='ddlMake']").val();
PageMethods.FillModel(makeid,onsuccess,onfailed);

}

function onsuccess(res)
{

var modelList=$("select[id$='ddlModel']");
var Version=$("select[id$='ddlVersion']");

Version.empty();
Version.append("<option value='dntSave_'>----NO Version----</option>")
modelList.empty();
modelList.append("<option value='dntSave_'>----Select----</option>");

var arrModel=res.split('#');
var leng=arrModel.length;
for(var i=0;i<leng-1;i++)
{
var newOption=arrModel[i].split(',');

modelList.append("<option value="+newOption[1]+">"+newOption[0]+"</option>");
}

}
function onfailed(res)
{
alert(res.get_message());
}

code on button click

protected void btn_Click(object sender, EventArgs e)
{

  string model=  ddlModel.SelectedValue.ToString();
 //every time it show model="dntSave_"
}