Why do ASP.NET JSON web services return the result in 'd'?

I wrote some ASP.NET web services that use JSON encoding, a la:

[WebInvoke()]
[OperationContract]
public int SetInformation(int recordid, string data)
{
    return 42;
}

and the returned JSON is:

{"d": 42}

Why is the parameter named d? Can I control that? Say, to e?

For reference, a few similar questions I've finally been able to dig up:

• What does .d in JSON mean?
• How to change ASP.NET WebMethod's 'd' identifier in a JSON response, to a different name?
• Returning HTML from JSON webservice - what is the ".d"?