Error in datagridview

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C#

C#4.0

Hi friends
The below code is for binding datasource to datagridview for three radiobutton checked event.

private void button6_Click(object sender, EventArgs e)
        {
            sdt.ACADAMICYEAR = textBox4.Text;
            sdt.CLASSID = Convert.ToInt32(comboBox1.SelectedValue.ToString());
            if (radioButton1.Checked == true)
            {
                
                DataTable studentbind = ibl4.studentbindforfemale(sdt);
                int rowcount = studentbind.Rows.Count;
                List<string> no = ibl4.setnoofint(rowcount);
                int registerno = ibl4.getregiter(sdt);
                dataGridView1.DataSource = studentbind;
               DataGridViewTextBoxColumn fourthcolumn = new DataGridViewTextBoxColumn();
                fourthcolumn.HeaderText = "RegNo";
                fourthcolumn.Name = "RegNo";
                
                dataGridView1.Columns.Add(fourthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    dataGridView1.Rows[i].Cells[3].Value = registerno;
                }

                DataGridViewComboBoxColumn fifthcolumn=new DataGridViewComboBoxColumn();
                fifthcolumn.HeaderText="RegNo";
                fifthcolumn.Name="RegNo";
                dataGridView1.Columns.Add(fifthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    fifthcolumn.DataSource = no;
                }
            }

            if (radioButton2.Checked == true)
            {
                
                    dataGridView1.Rows.Clear();
                
                dataGridView1.Refresh();
                DataTable studentbind = ibl4.studentbindformale(sdt);
                int rowcount = studentbind.Rows.Count;
                List<string> no = ibl4.setnoofint(rowcount);
                int registerno = ibl4.getregiter(sdt);
                dataGridView1.DataSource = studentbind;
                DataGridViewTextBoxColumn fourthcolumn = new DataGridViewTextBoxColumn();
                fourthcolumn.HeaderText = "RegNo";
                fourthcolumn.Name = "RegNo";

                dataGridView1.Columns.Add(fourthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    dataGridView1.Rows[i].Cells[3].Value = registerno;
                }

                DataGridViewComboBoxColumn fifthcolumn = new DataGridViewComboBoxColumn();
                fifthcolumn.HeaderText = "RegNo";
                fifthcolumn.Name = "RegNo";
                dataGridView1.Columns.Add(fifthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    fifthcolumn.DataSource = no;
                }
            }
            if (radioButton3.Checked == true)
            {
               
                 dataGridView1.Rows.Clear();
                
                dataGridView1.Refresh();
                DataTable studentbind = ibl4.studentbindforgeneral(sdt);
                int rowcount = studentbind.Rows.Count;
                List<string> no = ibl4.setnoofint(rowcount);
                int registerno = ibl4.getregiter(sdt);
                dataGridView1.DataSource = studentbind;
                DataGridViewTextBoxColumn fourthcolumn = new DataGridViewTextBoxColumn();
                fourthcolumn.HeaderText = "RegNo";
                fourthcolumn.Name = "RegNo";

                dataGridView1.Columns.Add(fourthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    dataGridView1.Rows[i].Cells[3].Value = registerno;
                }

                DataGridViewComboBoxColumn fifthcolumn = new DataGridViewComboBoxColumn();
                fifthcolumn.HeaderText = "RegNo";
                fifthcolumn.Name = "RegNo";
                dataGridView1.Columns.Add(fifthcolumn);
                for (int i = 0; i < rowcount; i++)
                {
                    fifthcolumn.DataSource = no;
                }
            }

        }

when i click signle radiobutton(any of three) for button event, it gives a expected result.but if i click the second radiobutton and it does add the result to already datagridvalue. for clearing datagridview i use datagridview.rows.clear(). but it raise an error as "

Cannot clear the list

". i want to display the appropriate value for appropriate radiobutton click event.
please find solve my problem.
Thank you