code: public void addAnswer(int questionId, int answerId) {
      QuestionAnswerPair answerPair = new QuestionAnswerPair(questionId, answerId);
      answers.add(answerPair);
    }
docs: Adds an answer to a question to the AnswerSet.
  
  @param questionId  the id of the question to be answered
  @param answerId  the id of the answer selected