select dropdwon list value change my url ???

i friend facing a problem :

i have one drop down list in which select my size then prize change according to size:

but my url is change why please solve this problem:

like:

first time click product show my url like this:

but when select size another then show my url like this:

dropdown list: like

and code:

protected void ddl_size_SelectedIndexChanged(object sender, EventArgs e)

{

if (ddl_size.SelectedValue != "")

{

Objadmin.Operationname = "Bindupdateprice";

Session["Image_Id"] = int.Parse(Image_Id.Value);

hid_productid.Value = Session["Image_Id"].ToString();

Objadmin.ImageId = int.Parse(Image_Id.Value);

Objadmin.TagName = Session["tagname"].ToString();

Objadmin.ProductCode = "";

Objadmin.Price = 0;

Objadmin.Size_Id = int.Parse(ddl_size.SelectedValue);

DataSet ds = Objadmin.ProductOperations();

if (ds != null && ds.Tables.Count != 0)

{

if (ds.Tables[0].Rows.Count != 0)

{

lblPrice.Text = Convert.ToString(ds.Tables[0].Rows[0]["dollar"]);

}

string size = ddl_size.SelectedValue;

}