我對諒解有疑問，如何從XML讀取數據。XML看起來像這樣：<PosXML version="7.2.0">    <ReadCardResponse>        <ReturnCode>1</ReturnCode>        <Card>            <Pan>222300******5062</Pan>            <Expires>****</Expires>            <CardName>MASTERCARD</CardName>            <CardSource>2</CardSource>        </Card>    </ReadCardResponse></PosXML>我已經從流中加載了XML：XDocument doc;using (Stream responseStream = httpResponse.GetResponseStream()){    doc= XDocument.Load(responseStream);}試過這個，但是不起作用：XElement returnCode = doc.XPathSelectElement("ReturnCode")