c# quiz application running score

I am developing a quiz application that keeps a running score for example an incorrect answer gives -1 and a correct answer gives +1. It works fine but i am having trouble with the final question i would like my "next" button to change to a "results" button on the form and then on click display a message box displaying a message containing the score and a message. How could i implement this button change only when the answer has been submitted on the last question as i am catching an exception at the minute

here is my code:

  int score = 0;
    int i = -1;
    int a = 0;

    string[] questions = new string[] 
    {
        "Question 1?", 
        "Question 2", 
        "Question 3",
        "Question 4", 
    };


    string[] answers = new string[] 
    {
        "incorrect","correct","incorrect","incorrect",
        "correct","incorrect","incorrect","incorrect",
        "incorrect","incorrect","incorrect","correct",
       "incorrect","incorrect","correct","incorrect"



    };

    string[] quizAnswers = new string[] { "correct", "correct", "correct", "correct" }; 


    public FrmQuestion2()
    {
        InitializeComponent();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        this.Hide();
        Tutorial3 t3 = new Tutorial3();
        t3.Show(); 
    }

    private void FrmQuestion2_Load(object sender, EventArgs e)
    {

    }

    private void Sbutton_Click(object sender, EventArgs e)
    {
        try
        {

            if (i < questions.Length)
                i++;
            txtQuestion.Text = questions[i];

            RadA.Text = answers[a];
            a++;
            RadB.Text = answers[a];
            a++;
            RadC.Text = answers[a];
            a++;
            RadD.Text = answers[a];
            a++;
            Sbutton.Visible = true;
            Sbutton.Enabled = true;
            Sbutton.Text = "Submit"; 
           // Subutton.Visible = true;
           // Subutton.Enabled = true;

        }
        catch (Exception ex)
        {
            MessageBox.Show("Only 4 questions available: Your Score is: " + score + " You need at least a score of 3 to progress");


        }
        if (score >= 3)
        {
            nxtbutton.Visible = true;
            nxtbutton.Enabled = true;

        }
        else if (score < 3)
        {
            nxtbutton.Visible = false;
            nxtbutton.Enabled = false;
            RetryButton.Visible = true;
            RetryButton.Enabled = true; 

        }

    }

    private void Subutton_Click(object sender, EventArgs e)
    {


        try
        {


            if (getSelected().Equals(quizAnswers[i]))
            {
                MessageBox.Show("Correct");
                score++;

                txtScore.Text = Convert.ToString("Score:" + score);
                Subutton.Enabled = false; 
                Subutton.Visible = false;
                Sbutton.Visible = true;
                Sbutton.Enabled = true;
                Sbutton.Text = "Next";

            }

            else
            {
                MessageBox.Show("Incorrect");
                score--;

                txtScore.Text = Convert.ToString("Score:" + score);
                Subutton.Enabled = false;
                Subutton.Visible = false;
                Sbutton.Visible = true;
                Sbutton.Enabled = true;
                Sbutton.Text = "Next";

            }

        }
        catch (Exception ex) 
        {
            MessageBox.Show("Please start the quiz"); 

        }
    }



    string getSelected()
    {
        if (RadA.Checked)
            return RadA.Text.ToString();
        if (RadB.Checked)
            return RadB.Text.ToString();
        if (RadC.Checked)
            return RadC.Text.ToString();
        if (RadD.Checked)
            return RadD.Text.ToString();
        return "";

    }

    private void RetryButton_Click(object sender, EventArgs e)
    {
        this.Hide();
        FrmQuestion2 q2 = new FrmQuestion2();
        q2.Show(); 
    }

thanks for your help