Delete Row in GridView based on DropDownList Selection in ASP.Net

This Way,

HTML:

<form id="form1" runat="server">
    <div>
        <cc1:ToolkitScriptManager runat="server">
        </cc1:ToolkitScriptManager>
        <asp:DropDownList ID="ddlMarks" runat="server" AutoPostBack="true" OnSelectedIndexChanged="Marks_SelectedIndexChanged">
            <asp:ListItem Value="0" Text="Select" />
            <asp:ListItem Value="1" Text="Poor" />
            <asp:ListItem Value="2" Text="Good" />
        </asp:DropDownList>
        <br />
        <asp:GridView ID="gvReason" runat="server" AutoGenerateColumns="false">
            <Columns>
                <asp:BoundField DataField="Marks" HeaderText="Marks" />
                <asp:BoundField DataField="Reason" HeaderText="Reason" />
            </Columns>
        </asp:GridView>
        <asp:Panel ID="pnlReason" runat="server">
            Reason
            <br />
            <asp:TextBox ID="txtReason" TextMode="MultiLine" Width="300" runat="server" /><br />
            <asp:Button ID="btnSave" runat="server" OnClick="SaveToGrid" Text="Save" />
            <asp:Button ID="btnCancel" Text="Cancel" runat="server" />
        </asp:Panel>
        <asp:LinkButton ID="lnkFake" Text="" runat="server" />
        <cc1:ModalPopupExtender ID="mpeReason" runat="server" PopupControlID="pnlReason"
            CancelControlID="btnCancel" BackgroundCssClass="modalpopup" TargetControlID="lnkFake"
            BehaviorID="mpeDemo">
        </cc1:ModalPopupExtender>
    </div>
    </form>

C#:

protected void Marks_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddlMarks.SelectedItem.Value == "1")
        {
            this.mpeReason.Show();
        }
        if (ddlMarks.SelectedItem.Value == "2")
        {
            this.gvReason.DataSource = null;
            this.gvReason.DataBind();
        }
    }

    protected void SaveToGrid(object sender, EventArgs e)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[2]{new DataColumn("Marks",typeof(string)),
                                            new DataColumn("Reason",typeof(string))});
        string marks = ddlMarks.SelectedItem.Text;
        string reason = txtReason.Text.Trim() == string.Empty ? string.Empty : txtReason.Text.Trim();
        dt.Rows.Add(marks, reason);
        this.gvReason.DataSource = dt;
        this.gvReason.DataBind();
    }

STYLE:

<style type="text/css">
        .modalpopup
        {
            background-color: #ADADAD;
            filter: Alpha(Opacity=70);
            opacity: 0.70;
            -moz-opacity: 0.70;
        }
    </style>