前台js:

function Run_btn_delete() {
var a = $("#S1").text();
$.ajax({
type: "post",
url: "storage_list.aspx/btn_delete",
data: "{'a':'" + a + "'}",
dataType: "json",
contentType: "application/json; charset=utf-8",
success: function (res) {
var obj = jQuery.parseJSON(res)
 lbl.innerHTML = res.d;
},
error: function (XMLHttpRequest, textStatus, errorThrown) {
alert("错误：" + errorThrown);
}
});
}

后台：

[WebMethod]
public static string btn_delete(string a)
{
string strsql = "delete from Furniture where id='" + a.Trim() + "'";
if (DBhelper.ExecuteCommand(strsql) >= 1)
{
return "删除成功！";
}
else
{
return "删除失败！";
}
}

不明白为什么是这一句  lbl.innerHTML = res.d; 才显示“删除成功！”，这个d是哪里来的？求高手解答