Why isn't why code working and how can I fix it?

var questions = [ 
  ['How many states are in the United States?', 50], 
  ['How many continents are there?', 7], 
  ['How many legs does an insect have?', 6] 
]; 
var right = []; 
var wrong = []; 
var correctAnswers = 0; 
var question; 
var answer; 
var response;

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

function buildList(arr) { 
  var listHTML = '<ol>'; 
    for( var i = 0; i < arr.length; i += i ) {  // The error is on this line.
      listHTML += '<li>' + arr[i] + '</li>'; 
    } 
      listHTML += '</ol>'; 
      return listHTML; 
  }

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

html = 'You got ' + correctAnswers + ' question(s) right.' 
html += '<h2>You got these questions correct:</h2>'; 
html += buildList(right); 
html += '<h2>You got these questions wrong:</h2>'; 
html += buildList(wrong);

print(html);