图形界面上，如果单选框没有操作，就提示“请选择”，应该如何写
public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }

      

        private void radioButton1_CheckedChanged_1(object sender, EventArgs e)
        {
            if (radioButton1.Checked)
            {
                label1.Text = "你选择了A";
            }
        }

        private void radioButton2_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton2.Checked)
            {
                label1.Text = "你选择了B";
            }
        }

        private void radioButton3_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton3.Checked)
            {
                label1.Text = "你选择了C";
            }
        }

        private void radioButton4_CheckedChanged(object sender, EventArgs e)
        {
            if (radioButton4.Checked)
            {
                label1.Text = "你选择了D";
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (radioButton3.Checked)
            {
                MessageBox.Show("恭