Exception Error Incorrect syntax near 'System.Web.UI.WebControls.DataControlFieldCell'. how can I resolve my error

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" EnableModelValidation="True" Width="220px"  >
 
            <Columns>
 

                                <asp:BoundField DataField="QUESTION_NO" HeaderText="Q_NO" />
                                <asp:BoundField DataField="QUESTION_DESCRIPTION" HeaderText="DESCRIPTION" />
 

 
                <asp:TemplateField HeaderText="ANSWERS">
                    <ItemTemplate>
                        <asp:DropDownList ID="DropDownListUser" runat="server" AutoPostBack="False">
                            <asp:ListItem Text="Yes" Value="True"></asp:ListItem>
                            <asp:ListItem Text="No" Value="False"></asp:ListItem>
                        </asp:DropDownList>
                    </ItemTemplate>
                </asp:TemplateField>
 

 
            </Columns>
 

 
        </asp:GridView>






string query;
               for (int i = 0; i < GridView1.Rows.Count - 1; i++)
               {
 
                   query = "insert into student_info values('" + regtxtbox.Text + "','" + gentxtbox.Text + "','" + bactxtbox.Text + "','" + pretxtbox.Text + "','" + gratxtbox.Text + "','" + Unetxtbox.Text + "','" + agetxtbox.Text + "','" + cgptxtbox.Text + "','" + smetxtbox.Text + "','" + sestxtbox.Text + "','" + exptxtbox.Text + "','" + martxtbox.Text + "'.'" + GridView1.Rows[i].Cells[0].ToString() + "','" + GridView1.Rows[i].Cells[2].ToString() + "')";
 
                   MessageBox.Show("SURVEY INFORMATION HAS BEEN STORED IN DATABASE");
 
                   SqlCommand cmd = new SqlCommand(query, conobj);
 
                   cmd.ExecuteNonQuery();
               }