Cómo persistir los datos del modelo cuando se publica un formulario

When form is showing initially then I show question and their answer. I show multiple radio button for answer. So user can select only one answer. The problem occurs when I submit my form and action method calls. I saw when form post then answer model is getting null. So guide me how to write code as a result answer model should not be null when form will post.

Here is full code. Please go through the code and if requires changes as a result answer model will not be null when form will be posted to action method.

my ViewModels code

namespace ViewModels
{
    public class Question
    {
        public int ID { set; get; }
        public string QuestionText { set; get; }

        public List<Answer> Answers { set; get; }
        [Required]
        public string SelectedAnswer { set; get; }

        public Question()
        {
            Answers = new List<Answer>();
        }
    }

    public class Answer
    {
        public int ID { set; get; }
        public string AnswerText { set; get; }
    }

    public class Evaluation
    {
        public List<Question> Questions { set; get; }
        public Evaluation()
        {
            Questions = new List<Question>();
        }
    }
}

controller code

 public ActionResult Index()
        {
            var evalVM = new Evaluation();

            //the below is hardcoded for DEMO. you may get the data from some  
            //other place and set the questions and answers

            var q1 = new Question { ID = 1, QuestionText = "What is your favourite language" };
            q1.Answers.Add(new Answer { ID = 12, AnswerText = "PHP" });
            q1.Answers.Add(new Answer { ID = 13, AnswerText = "ASP.NET" });
            q1.Answers.Add(new Answer { ID = 14, AnswerText = "Java" });
            evalVM.Questions.Add(q1);

            var q2 = new Question { ID = 2, QuestionText = "What is your favourite DB" };
            q2.Answers.Add(new Answer { ID = 16, AnswerText = "SQL Server" });
            q2.Answers.Add(new Answer { ID = 17, AnswerText = "MySQL" });
            q2.Answers.Add(new Answer { ID = 18, AnswerText = "Oracle" });
            evalVM.Questions.Add(q2);

            return View(evalVM);           
        }

 [HttpPost]
        public ActionResult Index(Evaluation model)
        {
            if (ModelState.IsValid)
            {
                foreach (var q in model.Questions)
                {
                    if(q.Answers==null)
                    {
                        // Answers is null
                    }
                    var qId = q.ID;
                    var selectedAnswer = q.SelectedAnswer;
                    // Save the data 

                }
                return RedirectToAction("ThankYou"); //PRG Pattern
            }
            //reload questions
            return View(model);
        }

index.cshtml code

@model ViewModels.Evaluation
<h2>Quiz 24</h2>
@using (Html.BeginForm())
{
    @Html.EditorFor(x=>x.Questions)
    <input type="submit" />
}

and view code which is stored in EditorTemplates folder

@model ViewModels.Question
<div>
    @Html.HiddenFor(x=>x.ID)
    <h3> @Model.QuestionText </h3>

    @foreach (var a in Model.Answers)
    {
        <p>
            @Html.RadioButtonFor(b=>b.SelectedAnswer,a.ID)  @a.AnswerText 
        </p>

    }
</div>

el problema esta aqui

 [HttpPost]
        public ActionResult Index(Evaluation model)
        {
            if (ModelState.IsValid)
            {
                foreach (var q in model.Questions)
                {
                    if(q.Answers==null)
                    {
                        // Answers is null
                    }
                    var qId = q.ID;
                    var selectedAnswer = q.SelectedAnswer;
                    // Save the data 

                }
                return RedirectToAction("ThankYou"); //PRG Pattern
            }
            //reload questions
            return View(model);
        }

q.Answers==null is getting null when i post the form. i like to know the trick that how to write code in such a way when form will be post to action then Answers should be null. many guy told me that i need to rebuild the model manually because Answers will be always null. is there any no mechanism in MVC to persist all the data and properly De-serialize it to model when form will be posted.