Watin multithreading issue

Question

I have a problem in my application, written in c# using WatiN. The application creates few threads,and each thread open browser and the same page.
The page consist of HTML select element:

and a submit button.
The browsers should select a specific option and click on the submit button at the same time but instead they do it "one by one". Here is the main code lines:

[STAThread]
static void Main(string[] args)
{ 

    for (int i = 0; i < numOfThreads;i++ )
    {
        var t = new Thread(() => RealStart(urls[i]));
        t.SetApartmentState(ApartmentState.STA);
        t.IsBackground = true;
        t.Start();
    }
}

private static void RealStart(string url)
    {
        using (var firstBrowser = new IE())
        {                
            firstBrowser.GoTo(url);
            firstBrowser.BringToFront();           
            OptionCollection options = firstBrowser.SelectList("Select").Options;
            options[1].Select();
            firstBrowser.Button(Find.ByName("Button")).Click();
            firstBrowser.Close();
        }
    }

What is the cause of the "one by one" selection instead of simultaneously selection?

Answer 1

Solution:
After a long research I gave up using WatiN for this porpuse.
Instead, I have created HttpWebRequest and post it to the specific URL.
Works Like a charm:

HttpWebRequest httpWReq = (HttpWebRequest)WebRequest.Create("http://domain.com/page.aspx");
ASCIIEncoding encoding = new ASCIIEncoding();
string postData = "username=user";
postData += "&password=pass";
byte[] data = encoding.GetBytes(postData);
httpWReq.Method = "POST";
httpWReq.ContentType = "application/x-www-form-urlencoded";
httpWReq.ContentLength = data.Length;
using (Stream stream = httpWReq.GetRequestStream())
{
   stream.Write(data,0,data.Length);
}
HttpWebResponse response = (HttpWebResponse)httpWReq.GetResponse();
string responseString = new StreamReader(response.GetResponseStream()).ReadToEnd();

I send those requests simultaneously, by creating a Thread for each request.