DropDownList Problem in ASP.NET

Hello

I have created one ASPX page on which 2 combo boxes (dropdownlist) are there.

Using CSV file values (A01, B01) are displayed in first combo box. Now depending on selection of value in first combo box, I am trying to display values (AA, AB, AC) in second combo box.

Pls suggest how to identify selection in dropdownlist ?

What is significance of
Sub Combo2_SelectedIndexChanged(sender As Object, e As EventArgs)


----------- Test01.CSV ------------
1,A01,1,AA
1,A01,2,AB
1,A01,3,AC
2,A01,1,AA
2,A01,2,AB
2,A01,3,AC
----------- Test01.CSV ------------

Dim csvPath As String = "C:\Inetpub\wwwroot\ASPdotNET\testAkita\"
Dim sCSVFileName As String ="Test01.csv"
Dim sr As StreamReader
Dim AL as ArrayList = new ArrayList()

Dim txtDelimiter as String = ","
Dim Temp1, Temp2 as String
Dim i as integer

Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)

If Page.IsPostBack = False Then

If File.Exists(csvPath & sCSVFileName) = True Then

Try
' Open the file to read from.

sr = File.OpenText(csvPath & sCSVFileName)
sr.ReadLine()
Temp1 = ""
Temp2 = ""
Do While sr.Peek() >= 0
Dim txtTemp=Split(sr.ReadLine(), txtDelimiter)
Temp1 = txtTemp(1)
If (Temp1 <> Temp2) Then
AL.Add(Temp1)
Temp2 = Temp1
End If
Loop
sr.Close()

ddlChiiki.DataSource = AL
ddlChiiki.DataBind()

Catch x As Exception
Response.Write(x.ToString())
End Try
End If
End If
End Sub