private void webBrowser_PreviewKeyDown(object sender, PreviewKeyDownEventArgs e)
        {
            if ((Control.ModifierKeys & Keys.Control) == Keys.Control)
            {
                switch (e.KeyValue)
                {
                    case (int)Keys.C:
                        webBrowser.Document.ExecCommand("Copy",true,null);
                        break;
                    case (int)Keys.V:
                        　IDataObject iData = Clipboard.GetDataObject(); 
　　                       if(iData.GetDataPresent(DataFormats.Text)) {
                              webBrowser.Document.ExecCommand("Paste", true, (String)iData.GetData(DataFormats.Text)); 
                          }
                        break;
                    case (int)Keys.X:
                        webBrowser.Document.ExecCommand("Cut", true, null);
                        break;
                    case (int)Keys.A:
                        webBrowser.Document.ExecCommand("SelectAll",true,null);
                        break;
                }
            }
        }