WebService with ResponseFormat.Json won't return JSON, only XML

I can't get my ASP.NET AJAX webservice to return JSON, it always returns XML.  The docs say that it should return JSON automatically -- how do I enable this?

 Here is my WebService.cs source:
1 using System;
2 using System.Web;
3 using System.Collections;
4 using System.Web.Services;
5 using System.Web.Services.Protocols;
6 using Microsoft.Web.Script.Services;
7
8 /// <summary>
9 /// Summary description for WebService
10 /// </summary>
11
12 [ScriptService]
13 public class WebService : System.Web.Services.WebService {
14
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