C# 从XML获取值

我有一个XML。如何从中获取CustomerSet值

<a:CreditCards>
 <a:CreditCard>
  <a:AccountNumber>1234XXXXXX23XXXX</a:AccountNumber> 
 <a:ExtendedInfo>
  <a:CustomerSet>RC808687</a:CustomerSet> 
 </a:ExtendedInfo>
 </a:CreditCard>
<a:CreditCards>

尝试使用XPath， 要获取CustomerSet值，xpath应该是/CreditCards/CreditCard/ExtendedInfo/CustomerSet

XmlDocument xml = new XmlDocument();
            xml.LoadXml("<CreditCards>  <CreditCard>   <AccountNumber>1234XXXXXX23XXXX</AccountNumber>   <ExtendedInfo>   <CustomerSet>RC808687</CustomerSet>   </ExtendedInfo>  </CreditCard> </CreditCards> ");
            XmlNodeList xmlList = xml.SelectNodes("/CreditCards/CreditCard/ExtendedInfo/CustomerSet");

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此XML格式不正确。未声明前缀a。请发布完整的XML。名称空间的声明在哪里？在示例XML中前缀为a:的XML名称空间如何？在示例XML中前缀为a:的XML名称空间如何？？