如何附加到 xml

我有这个xml。

<project>
   <user>
      <id>1</id>
      <name>a</name>
   </user>
   <user>
      <id>2</id>
      <name>b</name>
  </user>
 </project>

现在如何在元素之间附加这样的新元素<project></project>

<user>
   <id>3</id>
   <name>c</name>
</user>

string xml =
   @"<project>
        <user>
           <id>1</id>
           <name>a</name>
        </user>
        <user>
           <id>2</id>
           <name>b</name>
        </user>
     </project>";

XElement x = XElement.Load(new StringReader(xml));
x.Add(new XElement("user", new XElement("id",3),new XElement("name","c") ));
string newXml = x.ToString();