XPath Expression not working in HtmlAgilityPack

I know it may be of my noobness in XPath, but let me ask to make sure, cuz I've googled enough.

I have a website and wanna get the news headings from it: www.farsnews.com (it is Persian)

Using FireBug and FireXpath extensions under firefox and by hand I extract and test multiple Xpath expressions that matches the headings, such as:

* html/body/div[2]/div[2]/div[2]/div[*]/div[2]/a/div[2]
* .//*[@class="topnewsinfotitle "]
* .//div[@class="topnewsinfotitle "]

I also tested these using XPather extension and they seem to work pretty well, but when I get to test them... the SelectNodes returns null!

Any clue or hint?

here is a chunk of the code:

listBox2.ResetText();

HtmlAgilityPack.HtmlWeb w = new HtmlAgilityPack.HtmlWeb();
HtmlAgilityPack.HtmlDocument doc = w.Load("http://www.farsnews.com");
HtmlAgilityPack.HtmlNodeCollection nc = doc.DocumentNode.SelectNodes(".//div[@class=\"topnewsinfotitle \"]");

listBox2.Items.Add(nc.Count+" Items selected!");

foreach (HtmlAgilityPack.HtmlNode node in nc) {
            listBox2.Items.Add(node.InnerText);
        }

Thanks.