How to display hierarchical XML from a DataSet

I am trying to generate a XML using [WebMethod]..

Here is my webmethod in MyMethod.asmx.cs:

[WebMethod]
    public string GetTransactionList(string AccNo)
    {
        TransactionRepository transRep = new TransactionRepository();
        IList<Transaction> listTrans = transRep.GetTransactionList(AccNo);
        IList<Parameter> listPara = transRep.GetAllParameter();

        DataTable tblTrans = CommonDatatableMethods.ConvertToDataTable<Transaction>(listTrans, true);
        tblTrans.TableName = "transaction";
        tblTrans.Columns.Remove("SequenceNo");
        tblTrans.Columns.Remove("ID");

        /** Note: Require insert into dataset to gain custom root name**/
        DataSet dsTrans = new DataSet("Transactions"); 
        dsTrans.Tables.Add(tblTrans);

        StringWriter sw = new StringWriter();
        tblTrans.WriteXml(sw);

        if (listTrans.Count <= 0)
        {
            string emptydata = string.Empty;
            emptydata = "<Transactions><transaction>No record for " + AccNo + " within " + listPara[0].ParameterYears + " years data.</transaction></Transactions>";
            return emptydata;
        }
        else
        {
            /** Note: Need to remove all the schemas in meta tag**/
            return sw.ToString().Replace(" xsi:type=\"xs:string\" xmlns:xs=\"http://ift.tt/tphNwY\" xmlns:xsi=\"http://ift.tt/ra1lAU\"", "").Replace(" xml:space=\"preserve\"", "");
        }
    }

But when I click button invoke on my webservice,

The XML display only one line like this:

<Transactions><transaction><AccNo>ABC15279182719</AccNo><FirstName>AHMAD ALI</FirstName></transaction></Transactions>

What I want is something hierarchical like this:

<Transactions>
  <transaction>
    <AccNo>ABC15279182719</AccNo>
    <FirstName>AHMAD ALI</FirstName>
  </transaction>
</Transactions>

How do I make like that?

Thank You!