how to write the code for dropdownlist selected index changed

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C#

My source code as follows

<asp:GridView ID="gvfaculty" runat="server" AutoGenerateColumns="False"
                           BackColor="LightGoldenrodYellow" BorderColor="Tan" BorderWidth="1px"
                           CellPadding="2" ForeColor="Black" GridLines="None" Height="107px"
                           onrowcreated="gvfaculty_RowCreated" onrowdatabound="gvfaculty_RowDataBound"
                           onselectedindexchanged="gvfaculty_SelectedIndexChanged" Width="91px">

                          <asp:BoundField DataField="Faculty Name" HeaderText="Faculty Name" />
                          <asp:BoundField DataField="FacultyID" HeaderText="FacultyID" />
                          <asp:TemplateField HeaderText="1">
                            <itemtemplate>
                                       <asp:DropDownList ID="DropDownList1" runat="server" AppendDataBoundItems="true">
                                           <asp:ListItem Value="-1">--Select Grading --

                                   </itemtemplate>


                               <asp:TemplateField HeaderText="2">
                                   <itemtemplate>
                                       <asp:DropDownList ID="DropDownList2" runat="server" AppendDataBoundItems="true">
                                           <asp:ListItem Value="-1">--Select Grading --

                                   </itemtemplate>


                               <asp:TemplateField HeaderText="3">
                                   <itemtemplate>
                                       <asp:DropDownList ID="DropDownList3" runat="server" AppendDataBoundItems="true">
                                           <asp:ListItem Value="-1">--Select Grading --

                                   </itemtemplate>

for the above three dropdownlist how to write code for dropdownlist selected index changed event.

for that how can i do in asp.net using c#.