Here is my code:
<%@ Page Language=\"C#\" AutoEventWireup=\"true\" CodeBehind=\"Default.aspx.cs\" Inherits=\"WebClient._Default\" %>
Looks like the script tag for jquery is not closig properly unless you put a tag to close it, which renders those objects not readable, which gives you the error.
Code below, hope this helps.
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebClient._Default" %>