客户端用JavaScript填充DropDownList控件 服务器端读不到值

<form id="form1" runat="server">
<div>
<h3>看看用js填充的dropdownlist控件在服务器端能读出来吗？</h3>
三个级联下拉列表框：
<asp:DropDownList runat="server" id="bigTypeList" Width="150">
</asp:DropDownList>
<asp:DropDownList runat="server" id="typeList" Width="150">
</asp:DropDownList>
<asp:DropDownList runat="server" id="smalltypeList" Width="150">
</asp:DropDownList>
<br />
<asp:Button runat="server" Text="读取下拉表" ID="OK" onclick="OK_Click" /><br />
你选的是：<asp:Label runat="server" Text="Label" ID="label1"></asp:Label>
</div>
</form>