问SQLMVC2+ LINQ ASP.NET - CS0012编译错误

EN

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<dynamic>" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Index
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <h2>Index</h2>
    <% foreach (GameSite.Models.Category category in (IEnumerable)ViewData["Categories"])
       { %>
       <%= category.Name %>
       <% foreach (GameSite.Models.Forum forum in (IEnumerable)category.Forums)
          { %>
          <%= forum.Name %>
       <% } %>
    <% } %>
</asp:Content>

Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. 

Compiler Error Message: CS0012: The type 'System.Data.Linq.EntitySet`1<T0>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data.Linq, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

Source Error:    
Line 11:        { %>
Line 12:        <%= category.Name %>
Line 13:        <% foreach (GameSite.Models.Forum forum in (IEnumerable)category.Forums)
Line 14:           { %>
Line 15:           <%= forum.Name %>