两个下拉框数据互连问题。。。

当我点大类的时候，如果这个大类里有小类的话，小类下拉框激活。在不类框中显示大类的小类列表，如果没有小类不激活。
代码如下。数据库都写好了，就是不知道怎么让这两个下拉框互联起来。。

protected void Page_Load(object sender, EventArgs e)
{
OleDbConnection connstr = DBConnections.DBConnections.createConnection();
connstr.Open();

OleDbCommand cmd = new OleDbCommand();
cmd.Connection = connstr;
cmd.CommandText = "SELECT [BigID], [BigClassName] FROM [Hover_BigClass_New]";



DataSet daset = new DataSet();
OleDbDataAdapter ol = new OleDbDataAdapter();
ol.SelectCommand = cmd;
ol.Fill(daset);

if (daset.Tables.Count > 0)
{
if (daset.Tables[0].Rows.Count > 0)
{
ClassNews.Enabled = true;
ClassNews.AutoPostBack = true;
ClassNews.DataSource = daset;
ClassNews.DataTextField = "BigClassName";
ClassNews.DataValueField = "BigID";
ClassNews.DataBind();
if(Request["BigClassName"].ToString()!=null)
{
////////////小类
String BigClassNames = Request["BigClassName"].ToString();
OleDbCommand cmd1 = new OleDbCommand();
cmd1.Connection = connstr;
cmd1.CommandText = "Select * From Hover_SmallClass_New where BigClassName='" + BigClassNames + "'";

DataSet ds1 = new DataSet();
OleDbDataAdapter dsda = new OleDbDataAdapter();
dsda.SelectCommand = cmd1;
dsda.Fill(ds1);

if (ds1.Tables.Count > 0)
{
if (ds1.Tables[0].Rows.Count > 0)
{
SmallClassNews.DataSource = ds1;
SmallClassNews.Enabled = true;
SmallClassNews.DataTextField = "SmallClassName";
SmallClassNews.DataValueField = "SmallClassID";
SmallClassNews.DataBind();
}
else
{
SmallClassNews.Enabled = false;
}
}
else
{
SmallClassNews.Enabled = false;
}
ds1.Dispose();
ds1 = null;
　　　　　　　　　　///////////小类结束
}

}
else
{
ClassNews.Enabled = false;
}
}
else
{
ClassNews.Enabled = false;
}

ol.Dispose();
ol = null;

connstr.Close();

}
以下是前台代码
<asp:DropDownList ID="ClassNews" runat="server" Height="23px" Width="86px">
</asp:DropDownList>

　<asp:DropDownList ID="SmallClassNews" runat="server" Height="23px" Width="86px">
</asp:DropDownList>