Exam designer in CS

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;

namespace Exam_System
{
    public partial class frmQdesign : Form
    {
        public frmQdesign()
        {
            InitializeComponent();
        }

        private void frmQdesign_FormClosing(object sender, FormClosingEventArgs e)
        {
            Application.Exit();
        }

        private void btnExit_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            string sFname = txtFname.Text;
            string sQ = txtQ.Text;
            string sA1 = txtA1.Text;
            string sA2 = txtA2.Text;
            string sA3 = txtA3.Text;
            string sA4 = txtA4.Text;
            string sCA = txtCA.Text;

            string sQuestion = "\nN#" + sQ + "#" + sA1 + "#" + sA2 + "#" + sA3 + "#" + sA4 + "#" + sCA;

            if (File.Exists (sFname ))
            {
                using (StreamWriter sw = new StreamWriter(sFname,true ))
                {
                    sw.WriteLine(sQuestion );
                }
            }
            else
            {
                MessageBox.Show("Invalid File");
                txtFname.SelectAll();
                txtFname.Focus();
            }


        }
    }
}