Dropdownlist select 赋值取值问题

一，后台给dropdownlist添加请选择；

 ListItem li = new ListItem("请选择", " ");
            ListItem it = new ListItem();
            it.Text = "请选择";
            it.Value = "";
            it.Selected = true;
            dropdownlist.Items.Add(it);

 二，前台给select 加请选择及赋值

$("#objSelect").attr("value", "请选择");

 三，动态给select添加下拉列

for (var i = 0; i < strJson.length; i++) {
                        var option = $("<option>").text(strJson[i].CheXi).val(strJson[i].CheXi);
                        $("#tdplCheXi").append(option);
                    }