html中设置让div中的内容超出后自动显示滚动条

<div class="classlist">
<div class="autoScroll">
<asp:RadioButtonList ID="rblClasses" runat="server" DataTextField="className" DataValueField="classID"
ForeColor="#FF4040">
</asp:RadioButtonList>

<asp:ImageButton ID="imgSure" runat="server" ImageUrl="~/images/c_sure.jpg" />
<asp:ImageButton ID="imgClose" runat="server" ImageUrl="~/images/c_close.jpg" />
</div>
</div>

<style type="text/css">
.classlist
{
width: 300px;
height: 20px;
}
.autoScroll
{
width:300px;
height:100px;
overflow:auto;
}
#rblClasses
{
margin-left: 20px;
}
</style>