I am receiving this error : undefined is not an object

function print(message) {
  document.write(message+'<br>');
}

function promptMe(qa1){
  ans = prompt(qa1[0]);
  if( ans === qa1[1]){
    prompt("Your are right!!");
    return 1;
  }
  return 0;
}

function quizMe(qas){
  correct_ans = []
  for(var i=0;i< qas.length ; i++) {
    res = promptMe(qas[i]);
    if( res !== 0) {
      correct_ans.push(i);
    }
  }
  return correct_ans;
}

function main_f() {
  var qas = [ ["What is the main tag for a html page?", "html"],
          ["How many States has the Unites States?", "50"],
          ["Does exist Alien?", "Yes"]];
  var res = quizMe(qas);

  print("Right Answered questions");
  for(var i=0; i < res.length ; i++) {
    print(qas[res[i]][0]);    
    print(qas[res[i]][1]);
  }
  print("Wrong Answered questions");
  var count_match=0;
  for(var j=0; j < qas.length ; j++) {
    if(res.indexOf(j)===-1){
      print(qas[res[j]][0]); //<<---Undefined is not an object(evaluating 'qas[res[j]]')   
      print(qas[res[j]][1]);
    }
    }    

}

main_f()