Refference Error: Variable not defined

var quiz = [
["What is the main religion of Israel?", "Judaism"], 
["What word stands for 'truth' in Hebrew?", "Emet"], 
["Is Donald Trump an idiot?", "Yes"] 
] ;

var correctAnswers = 0;

var correctQuestions = [];
var wrongQuestions = [];

function print(message) {
  var outputDiv = document.getElementById("output");
  outputDiv.innerHTML = message;
}

for ( i = 0; i < 3; i += 1)  {
    var userAnswer = prompt(quiz[i][0]);
    if ( userAnswer.toUpperCase() === quiz[i][1].toUpperCase()) {
        correctAnswers += 1;
        correctQuestions.push(quiz[i][0]);
        alert("That's correct!");
    }

    else {
        alert("Wrong, correct answer was '" + quiz[i][1] + "'")
        wrongQuestions.push(quiz[i][0]);
    }
}

print("You answered " + correctAnswers + " correctly");
print("Here are the questions you answered correctly:" + "<ol><li>" + correctQuestions[0] + "</li>" + "<li>" + correctQuestions[1] + "</li>" + "<li>" + correctQustions[2] + "</li></ul>");
print("Here are the questions you didn't answer correctly:" + "<ol><li>" + wrongQuestions[0] + "</li>" + "<li>" + wrongQuestions[1] + "</li>" + "<li>" + wrongQustions[2] + "</li></ul>");