I'm done with the challenge, but I just can't seem to get rid of the 'undefined'. See my snapshot.

let questions = [
    ["How many stars in the galaxy", '1'],
    ["What's my mums name?", "frank"],
    ["How much of a dum dum are you out of 10?", '10']
];

// 2. Store the number of questions answered correctly
let correct_answers = 0;
let questions_passed = [];
let questions_failed = [];

/* 
  3. Use a loop to cycle through each question
      - Present each question to the user
      - Compare the user's response to answer in the array
      - If the response matches the answer, the number of correctly
        answered questions increments by 1
*/
for ( let i = 0; i < questions.length; i++ ) {
    let guess = prompt(`Hi there, ${questions[i][0]}`);
    let answer = questions[i][1]

    if ( guess === answer ) {
        alert("Correctamundo!");
        correct_answers++;
        questions_passed.push(questions[i][0]);
    } else {
        alert("Sorry that's incorrect. Moving on...");
        questions_failed.push(questions[i][0]);
    }

    document.querySelector('main').innerHTML = createOl(questions_failed);
}
function createOl(arr) {
    let ordered_list;
    for ( let i = 0; i < arr.length; i++ ) {
        ordered_list += `<li>${arr[i]}</li>`;
    }
    return ordered_list;
}

// 4. Display the number of correct answers to the user
document.querySelector('main').innerHTML = `
    <h1>Congratulations! You got ${correct_answers} correct!</h1>
    <h2>Questions you got right. :)</h2>
    <ol>${createOl(questions_passed)}</ol>
    <h2>Questions you got wrong. :(</h2>
    <ol>${createOl(questions_failed)}</ol>
`;