Query XML by select an element in CSharp

/*from www.  j a  va  2  s.  c  om*/
using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        XDocument customers =
        new XDocument(new XDeclaration("1.0", "UTF-8", "yes"),
                      new XDocumentType("Books", null, "Books.dtd", null),
                      new XProcessingInstruction("Book", "out-of-print"),
                      new XElement("Books",  new XElement("Book", new XAttribute("type", "Author"),
                         new XElement("FirstName", "J"),
                         new XElement("LastName", "R")),
                       new XElement("Book",
                         new XAttribute("type", "Author"),
                         new XElement("FirstName", "E"),
                         new XElement("LastName", "B"))));
        var queryResult = from c in customers.Elements() select c.Name;
        foreach (var item in queryResult)
        {
            Console.WriteLine(item);
        }
    }
}