having issue in extracting an element from xml

I have a xml like this :

  <?xml version="1.0" encoding="UTF-8"?>
    <InsertReceipts> 
 <receipts xmlns:a="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
     <a:string> 
     <![CDATA[ test]]>
      </a:string>
     </receipts> 
     </InsertReceipts>
 

I want to extract element a:string from this xml,but can't figure out to do this?

as i use XmlElement foo = (XmlElement)doc.SelectSingleNode("//a:string");

it gives a parsing error

Feedback and help appreciated

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