parse xml that contain colon

I have this XML text:

<test:solution xmlns:test="http://www.test.com/">
    <script/>
    <test:question>
        <test:param name="name1">value 1</test:param>
        <test:param name="name2"> value 2</test:param>
    </test:question>
</test:solution>

when I run my application that contains those lines:

XmlDocument doc = new XmlDocument();
doc.Load(xmlUrl);
XmlNode testQuestions = doc.SelectSingleNode("/test:solution/test:question");
//XmlNodeList testParamNodeList = testQuestions.SelectNodes("test:param");

It give an error, I know that the error come from the colons, but I don't know how to resolve it.

Thanks for help