asp.net中怎么清除dropdownlist中的项目

页面代码

<form id="form1" runat="server">
<div>

<asp:DropDownList ID="DropDownList1" runat="server">
<asp:ListItem>aaa</asp:ListItem>
<asp:ListItem>bbb</asp:ListItem>
<asp:ListItem>ccc</asp:ListItem>
<asp:ListItem>ddd</asp:ListItem>
</asp:DropDownList>
</div>
<asp:Button ID="Button1" runat="server" Text="a" OnClick="Button1_Click" />
<asp:Button ID="Button2" runat="server" Text="b" OnClick="Button2_Click" />
</form>

cs 文件代码
protected void Button1_Click(object sender, EventArgs e)
{
DropDownList1.Items.Clear();
}
protected void Button2_Click(object sender, EventArgs e)
{
D