Both of the suggestions in the comments above are valid solutions. I thought I would add this though.
You could use the SelectedIndexChanged event for each dropdownlist to make visible or enable the following dropdownlists. It would look something like this:
protected void dropdownlist_SelectedIndexChanged(object sender, EventArgs e)
{
dropdownlist2.Visible = true;
dropdownlist2.Enabled = true;
}