dropdownlist selects first value automatically

//on page load event
if (!Page.IsPostBack)
       {
           FillClass();
       }

public void FillClass()
    {
        cn.Close();
        cn.Open();
        string com = "Select * from Class";
        SqlDataAdapter adpt = new SqlDataAdapter(com, cn);
        DataTable dt = new DataTable();
        adpt.Fill(dt);
        DropDownList1.DataSource = dt;
        DropDownList1.DataTextField = "classname";
        DropDownList1.DataValueField = "srno";
        DropDownList1.DataBind();
        DropDownList1.Items.Insert(0, new ListItem("--Select--", "0"));
        cn.Close();
    }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {       
            cn.Close();
            cn.Open();
            string com = "Select * from SubjectName where std='" + DropDownList1.SelectedItem.Text + "'";
            SqlDataAdapter adpt = new SqlDataAdapter(com, cn);
            DataTable dt = new DataTable();
            adpt.Fill(dt);
            DropDownList2.DataSource = dt;
            DropDownList2.DataTextField = "subject";
            DropDownList2.DataValueField = "srno";
            DropDownList2.DataBind();
            DropDownList2.Items.Insert(0, new ListItem("--Select--", "0"));
            cn.Close();        
    }
protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
    {

       
            cn.Close();
            cn.Open();
            string com = "Select * from Chapter where std='" + DropDownList1.SelectedItem.Text + "' and subject='" + DropDownList2.SelectedItem.Text + "'";
            SqlDataAdapter adpt = new SqlDataAdapter(com, cn);
            DataTable dt = new DataTable();
            adpt.Fill(dt);
            DropDownList3.DataSource = dt;
            DropDownList3.DataTextField = "chapter";
            DropDownList3.DataValueField = "srno";
            DropDownList3.DataBind();
            DropDownList3.Items.Insert(0, new ListItem("--Select--", "0"));
            cn.Close();
        }
 
    protected void Button1_Click(object sender, EventArgs e)
    {
        Label1.Text = DropDownList3.Text;       
    }

<%@ Page Title="" Language="C#" MasterPageFile="~/admin/AdminMasterPage.master" AutoEventWireup="true" CodeFile="papergenr.aspx.cs" Inherits="admin_papergenr" %>

<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">

</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
        onselectedindexchanged="DropDownList1_SelectedIndexChanged">        
    </asp:DropDownList>
    

    <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" 
        onselectedindexchanged="DropDownList2_SelectedIndexChanged">
    </asp:DropDownList>


    <asp:DropDownList ID="DropDownList3" runat="server">
    </asp:DropDownList>



    <br />
   
 
 <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    <br />
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />

</asp:Content>