i am getting error in using dataset ds ,Can anybody tell how to use dll select cluase in code

public string Open(string sql)
        {
            try
            {
                //if (con.State != System.Data.ConnectionState.Open)
                if (!Connect())
                    return "Database connection error";

                cmd = new OleDbCommand(sql, con);
                da = new OleDbDataAdapter(sql, con);
                DataSet ds = new DataSet();
                da.Fill(ds);
                return Convert.ToString(ds);
               // return "sucessfully open";

            }
            catch (Exception ex)
            {
                return ex.ToString();

            }
        }

string sql = "SELECT Part_No, No_Of_Teeth, [Module], PA_Deg, PA_Min, PA_Sec, HA_Deg, HA_Min, HA_Sec, Od, Corr_Fact_X, Ball_Pin_Dia, Dimm_Over_PB, Span_Tooth,Span_Measurement, Act_DimOverBall FROM GearData";
     
           string ds = db.Open(sql);
           for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
               cmbprtno.Items.Add(ds.Tables[0].Rows[i][0].ToString());
           cmbprtno.Text = Convert.ToString(ds.Tables[0].Rows[0]["Part_No"].ToString());