how to use value of "ajax pro method " in button_click

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ASP.NET

hi to all.

i am binding dropdownlist in javascript through ajax pro method in code behind, now i want it's selected value in code behind.

Problem is that value is become null when i click save button in code behind. here is the code

  [AjaxPro.AjaxMethod]
      public DataSet  GetServerTime(DataSet ds)
      {
          con.Open();
          SqlDataAdapter adp = new SqlDataAdapter("select M_code,M_name from ICA", con);
          ds = new DataSet();
          adp.Fill(ds);
          return ds;

      }

 // now i am binding this in javascript :

   function getServerTime(res) {

       res = _Default.GetServerTime();
       var list =document.getElementById('<%= Drpscore.ClientID%>');

       for(var i=0; i < res.value.Tables[0].Rows.length; i++)
       {
         opt = new Option(res.value.Tables[0].Rows[i].M_name,res.value.Tables[0].Rows[i].M_code);
         list.options.add(opt);
       }

      }

  so now i have a button that i use to insert. the problem is that dropdown selected value not appear.

   protected void Button1_Click(object sender, EventArgs e)
      {
          string p = "";
          string t = Drpscore.selectedvalue;



      }

//  here t is coming null