delphi 添加xml cdata

  doc := LoadXMLData('<?xml version="1.0" encoding="GBK"?><root/>');
  doc.Active := true;
  root := doc.DocumentElement;

   _node:=root.AddChild('a');

   vNode:=doc.CreateNode('abc',ntCData,'xxxx');
     _node.ChildNodes.Add(vNode);