How do I get this program to pick the question randomly and then keep picking new random questions?

var questions = [
  ["Taxonomy is the __________ of organisms.","structure","classification","dissecting","opening","B"],
  ["How many charactersitics do all living things have in common?", "7","4","6","3","A"],
  ["Which of the following is not a taxanomic classifcation?", "kingdom", "height", "phyla","class","B"],
  ["Various species are located and classified in an ____________.", "habitat","chart","ecosystem","group","C"],
  ["The three main symbiotic relationships are: Mutalism, Paramatism and ____________.", "commensalism","existentialism","communism","pragmatism","A"]
];

var questionNumber = 1;
function randomNumber(){
  return Math.floor(Math.random() * questions.length);
}

function renderQuestion() {
  var test = document.getElementById("test");
  var num = randomNumber();
  test.innerHTML = "<div id='question'>"+questionNumber +") "+questions[num][0]+"</div>";
  test.innerHTML += "<button onclick='checkAnswer()' value='A'>"+"(a) "+questions[num][1]+"</button>"+"</br>";
  test.innerHTML += "<button onclick='checkAnswer()' value='B'>"+"(b) "+questions[num][2]+"</button>"+"</br>";
  test.innerHTML += "<button onclick='checkAnswer()' value='C'>"+"(c) "+questions[num][3]+"</button>"+"</br>";
  test.innerHTML += "<button onclick='checkAnswer()' value='D'>"+"(d) "+questions[num][4]+"</button>"+"</br>";
  questionNumber ++;
}

renderQuestion();

function checkAnswer(value) {
  renderQuestion();
}