DeSerializing XML data

I'm having trouble taking data from an XML file and reading it into my program. Here's the code I'm trying to use:

_xmlPath = NSBundle.MainBundle.PathForResource ("Questions", "xml");

using (var reader = new StreamReader (_xmlPath)) 
{
  var serializer = new XmlSerializer (typeof(Questions));
  var questionData = (Questions)serializer.Deserialize (reader);
}

The error I receive is at the line: var questionData = (Questions)serializer.Deserialize (reader); and the error I receive is System.InvalidOperationException was thrown. There is an error in XML document.

I'm trying to populate a Questions class which is as follows:

[Preserve(AllMembers = true)]
public class Questions
{
  public string QuestionTitle {get;set;}
  public List<string> Answers { get; set; }
}

Here is a snippet example of my XML file:

<?xml version="1.0" encoding="utf-8" ?>
<topic>
    <question>Question 1</question>
    <answers>
        <response>Answer 1</response>
        <response>Answer 2</response>
        <response>Answer 3</response>
        <response>Answer 4</response>
    </answers>
</topic>
<topic>
    <question>Question 2</question>
    <answers>
        <response>Answer 1</response>
        <response>Answer 2</response>
    </answers>
</topic>

What I'm trying to do is read the file into a List where each topic in the XML would represent an index in the List. For example:

questionList[0].question -> would return "Question 1"
questionList[1].answers[1].response -> would return "Answer 2"
etc.