急！！坐等答案。ListBox控件编程题

private void btn1_Click(object sender, EventArgs e)
{
if (listBox1.SelectedItem != null)
{
listBox2.Items.Add(listBox1.SelectedItem);
listBox1.Items.Remove(listBox1.SelectedItem);
}
}
private void btn2_Click(object sender, EventArgs e)
{
if (listBox2.SelectedItem != null)
{
listBox1.Items.Add(listBox2.SelectedItem);
listBox2.Items.Remove(listBox2.SelectedItem);
}
}

好好考试！不要百度我出的题

看不清楚！