webbrowser获取select已选中的值

HTML代码如下：

<select name="appVhlDTO.cNewMrk" id="cNewMrk" class="jquery-validatebox validatebox-text" required="true" readonly="true">
    <option value="">请选择</option>
    <option value="0" selected="">已上牌</option>
    <option value="1">未上牌</option>
</select>

C#代码如下：

HtmlElement selectHtmlElement = web.GetElementById(document, "cNewMrk");
int selectedIndex = Convert.ToInt32(selectHtmlElement.GetAttribute("selectedIndex"));
return selectHtmlElement.GetElementsByTagName("option")[selectedIndex].OuterText;