XML Parsing Error: Junk After Document Element

I got this error, XML Parsing Error: Junk After Document Element. My XMl was like this,

<?xml version="1.0" encoding="iso-8859-1" ?>
<billing_id>1234</billing_id>
<f_name>adam</f_name>

After some searching, I knew why,

A well-formed XML document must have one, and only one, top-level element. Anything after that element's end-tag will be ignored at best, and possibly reported as an error such as the one you're getting here.

So I fixed the problem by adding a top-level element <rsp>

<?xml version="1.0" encoding="iso-8859-1" ?>
<rsp>
  <billing_id>1234</billing_id>
  <f_name>adam</f_name>
</rsp>