asp.net两级联动（包含添加和修改）

<option value="x5" <%=GradeSelect == "x5" ? "selected":"" %>>五年级</option>
<option value="x6" <%=GradeSelect == "x6" ? "selected":"" %>>六年级</option>
<option value="c5" <%=GradeSelect == "c5" ? "selected":"" %>>七年级</option>
<option value="c6" <%=GradeSelect == "c6" ? "selected":"" %>>八年级</option>
<option value="c7" <%=GradeSelect == "c7" ? "selected":"" %>>九年级</option>
<option value="g1" <%=GradeSelect == "g1" ? "selected":"" %>>高一</option>
<option value="g3" <%=GradeSelect == "g3" ? "selected":"" %>>高二</option>
<option value="g4" <%=GradeSelect == "g4" ? "selected":"" %>>高三</option>
</select>
<select name="Course" id="Course" class="input">
<option value="0">请选择年级</option>
</select>
</body>
//C#代码
protected void Page_Load(object sender, System.EventArgs e)
{
public string GradeSelect = String.Empty;
public string SubjectSelect = String.Empty;
if (!(IsPostBack))
{
GradeSelect = Request.QueryString["Grade"];
SubjectSelect = Request.QueryString["Course"];
}
}