Visual Studio .NET - dynamic dropdownlist control

Hi 

    in my code for aspx i create a dropdown list dynamically and add it to a place holder control ,but upon postback i am unable to find the control or find the selected value in the dropbox so that i can create another dropbox with some other values.Here is the code 
 how do i solve this issue ? i tried adding the control again after postback

Please help 


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Dim dd1 As DropDownList = New DropDownList
Dim dd2 As DropDownList = New DropDownList

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        If Me.IsPostBack = True Then
           ' PlaceHolder1.Controls.Add(dd1)
           ' Dim dd2 As DropDownList = CType(Me.Placeholder1.FindControl("dd1"), DropDownList)
            If dd1.SelectedIndex = 2 Then
                dd2.Items.Add("None")
                dd2.Items.Add("B1")
                dd2.Items.Add("B2")
                PlaceHolder1.Controls.Add(dd2)
            End If
        Else
            dd1.Items.Add("None")
            dd1.Items.Add("A1")
            dd1.Items.Add("A2")
            dd1.AutoPostBack = True
            dd1.EnableViewState = False
            PlaceHolder1.Controls.Add(dd1)

        End If
    End Sub
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