how do I search for an atribute in a XML file?

Hello guys, I'm trying to seach for all the tags that have a expecific attribute, with no luck any help?

My XML file is something like this:

 <questions>
     <question id = "1" topic = "Science" language = "EN">
         <ask>test</ask>
         <answer1>test</answer1>
         <answer2>test</answer2>
         <answer3>test</answer3>
         <answer4>test</answer4>
         <rightOne>4</rightOne>
     </question>
      (...)

I only know how to get the information this way:

     public void SetupQuestion(int identifier){
             questions[identifier-1].TryGetValue("ask", out ask);
             questions[identifier-1].TryGetValue("answer1", out answer1);
             questions[identifier-1].TryGetValue("answer2", out answer2);
             questions[identifier-1].TryGetValue("answer3", out answer3);
             questions[identifier-1].TryGetValue("answer4", out answer4);
             questions[identifier-1].TryGetValue("rightOne", out rightOne);
             questions[identifier-1].TryGetValue("topic", out topic);
             questions[identifier-1].TryGetValue("id", out id);
     }

But how do I get all the questions by topic instead of by the order that they are placed in the document?

Thanks for your time!