undefined popping up after loop printed

var quizList= [
  ["What color is produced by mixing blue and yellow?", "green"],
  ["What color is produced by mixing blue and red?", "purple"],
  ["What color is produced by mixing yellow and red?", "orange"],
  ["What color is produced by mixing green and blue?", "aqua"]
];

var correctAnswer=0;
var wrongAnswer=0;

var questionCorrect;
var questionWrong;

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

for (i = 0; i < quizList.length; i += 1){
  var answer = prompt(quizList[i][0]);
  answer = answer.toLowerCase();
   if (answer === quizList[i][1]){
    correctAnswer += 1;  
    questionCorrect += "<p>" + quizList[i][0] + "</p>";
    } else {
    wrongAnswer += 1;  
    questionWrong += "<p>" + quizList[i][0] + "</p>";
    } 
  }

document.write("<h2> You got " + correctAnswer + " question\(s\) out of "  + quizList.length + " right. </h2>");

document.write("<h2> Questions you got right: </h2> " + questionCorrect);

document.write("<h2> Questions you got wrong: </h2>" + questionWrong);