reCaptcha和.NET出现问题

我在reCaptcha中遇到了以下错误:

'Input error: response: Required field must not be blank
challenge: Required field must not be blank
privatekey: Required field must not be blank'

    public static Boolean Check(String challenge, String response)
    {
        try
        {
            String privatekey = "7LeAbLoSAAAABJBn05uo6sZoFNoFnK2XKyF3dRXL";
            String remoteip = HttpContext.Current.Request.UserHostAddress;

            WebRequest req = WebRequest.Create("http://api-verify.recaptcha.net/verify");
            req.Method = "POST";

            using (StreamWriter sw = new StreamWriter(req.GetRequestStream()))
            {
                sw.Write("privatekey={0}&remoteip={1}&challenge={2}&response={3}", privatekey, remoteip, challenge, response);
                sw.Flush();
            }

            String resultString = String.Empty;
            String errorString = String.Empty;
            using (StreamReader sr = new StreamReader(req.GetResponse().GetResponseStream()))
            {
                resultString = sr.ReadLine();
                errorString = sr.ReadLine();
            }

            Boolean b;
            return Boolean.TryParse(resultString, out b) && b;
        }
        catch (Exception)
        {
            return false;
        }
    }

(当然那不是正确的私钥:P)

干杯。