请教一个错误问题“不能在DropDownList中选择多个项”

        protected void Page_Load(object sender, EventArgs e)
        {
            initDropList();
            BindGrid();
        }        private void initDropList()
        {
            for (int i = 2006; i < 2010; i++)
            {
                ListItem LIYear = new ListItem();
                LIYear.Text = i.ToString();
                LIYear.Value = i.ToString();
                if (i == DateTime.Today.Year)
                {
                    LIYear.Selected = true;
                }
                SYearBegin.Items.Add(LIYear);
                SYearEnd.Items.Add(LIYear);            }
            for (int i = 1; i < 13; i++)
            {                ListItem LIMonth = new ListItem();
                LIMonth.Text = i.ToString();
                LIMonth.Value = i.ToString();
                if (i == DateTime.Today.Month)
                {
                    LIMonth.Selected = true;
                }
                SMonthBegin.Items.Add(LIMonth);
                SMonthEnd.Items.Add(LIMonth);
            }            for (int i = 1; i <= DateTime.DaysInMonth(DateTime.Today.Year, DateTime.Today.Month); i++)
            {
                ListItem LIDay = new ListItem();
                LIDay.Text = i.ToString();
                LIDay.Value = i.ToString();
                if (i == DateTime.Today.Day)
                {
                    LIDay.Selected = true;
                }
                SDayBegin.Items.Add(LIDay);
                SDayEnd.Items.Add(LIDay);
            }
            ListItem L = new ListItem();
            L.Text = "选择";
            L.Value = "";
            L.Selected = true;
            SVacation.Items.Add(L);
            ArrayList vacationList = Vacation.RetrieveValidVacation();
            foreach (Vacation vacation in vacationList)
            {
                ListItem LI = new ListItem();
                LI.Text = vacation.VacationName;
                LI.Value = vacation.ID.ToString();
                SVacation.Items.Add(LI);
            }        }