Why am I getting an `undefined` message in my preview?

questions = [
  {
    question: 'What is 25 x 2?',
    answer: 50
  },
  {
    question: 'How many continents are there?',
    answer: 7
  },
  {
    question: 'How many legs does an insect have?',
    answer: 6
  }
];

function buildList (arr) {
  var listHTML = '<ol>';
  for (var i = 0; i < arr.length; i += 1) {
    listHTML += '<li>' + arr[i] + '</li>';
  }
  listHTML += '</ol>';
  return listHTML;
}

var correctAnswers = 0;
var question;
var answer;
var response;
var html;
var correct = [];
var wrong = [];

function print(message) {
  document.write(message);
}

for (var i = 0; i < questions.length; i += 1){
  question = questions[i].question;
  answer = questions[i].answer;
  response = parseInt(prompt(question));
  if (response === answer) {
    correctAnswers += 1;
    correct.push(question);
  } else {
    wrong.push(question);
  }
}

  html += '<p>You got ' + correctAnswers + ' questions right.</p>';
if (correctAnswers === 3) {
  html += '<h2>You got these questions right</h2>';
  html += buildList(correct);
} else if ( correctAnswers === 0){
  html += '<h2>You got these questions wrong</h2>';
  html += buildList(wrong);
} else {
  html += '<h2>You got these questions right</h2>';
  html += buildList(correct);
  html += '<h2>You got these questions wrong</h2>';
  html += buildList(wrong);
}
print(html);