protected void CheckBox1_CheckedChanged(object sender, EventArgs e)
    {
        WebService ser = new WebService();
        //为A调用的service方法,为了简单,暂时只返回一个值
        string result = ser.GetValue(A.SelectedValue);

        B.Items.Clear();
        B.Items.Add(new ListItem(result));
    }

        window.onload = function () {
            var selectA = document.getElementById("A");

            //假设A的第一行为空,
            if (selectA.selectedIndex == 0) {
                return;
            }

            var aValue = selectA[selectA.selectedIndex].value;

            WebService.GetValue(aValue, function (result) {
                var selectB = document.getElementById("B");
                selectB.innerHTML = "";

                var option = document.createElement("option");
                option.innerText = result;
                selectB.appendChild(option);
            });
        }