Вызов функций из js, но это не работает

function cs() {
  if (document.getElementById("cs").checked == true) {
    document.getElementById("isco").style.display = "none";
    document.getElementById("cnco").style.display = "none";
  }
  document.getElementById("csco").style.display = "initial";
}
document.getElementById("cs").onclick = cs

function is() {
  if (document.getElementById("is").checked == true) {
    document.getElementById("csco").style.display = "none";
    document.getElementById("cnco").style.display = "none";
  }
  document.getElementById("isco").style.display = "initial";
}
document.getElementById("is").onclick = is

function cn() {
  if (document.getElementById("cn").checked == true) {
    document.getElementById("isco").style.display = "none";
    document.getElementById("csco").style.display = "none";
  }
  document.getElementById("cnco").style.display = "initial";
}
document.getElementById("cn").onclick = cn

<div style = "background-color: white; text-align: left; padding-left:100px; padding-bottom: 10px">
<br><br><br>
<h3>Sign up</h3>

<form id = "sign" name = "sign" method = "POST" action = "sign.php">

  <label for = "major">Major:</label>
  <input type = "radio" name = "major" id = "cs" value = "cs">CS
  <input type = "radio" name = "major" id = "is" value = "is">IS
  <input type = "radio" name = "major" id = "cn" value = "cn">CN


  <div style = "display:  none;" id = "csco">
    <label for = "courses">Select the courses you finished or takes currently: </label>
    <br><br>
    <input type = "checkbox" name = "courses" value = "pr">Professional Responsibility
    <input type = "checkbox" name = "courses" value = "se">Software Engineering
    <input type = "checkbox" name = "courses" value = "alg">Analysis and Design of Algorithms
    <input type = "checkbox" name = "courses" value = "web">Web-based Systems
    <br>
  </div>
  <div style = "display: none;" id = "isco">
    <label for = "courses">Select the courses you finished or takes currently: </label>
    <br><br>
    <input type = "checkbox" name = "courses" value = "web">Web-based Systems
    <input type = "checkbox" name = "courses" value = "sad">System Analysis and Design(2)
    <br>
  </div>
  <div style = "display: none;" id = "cnco">
    <label for = "courses">Select the courses you finished or takes currently: </label>
    <br><br>
    <input type = "checkbox" name = "courses" value = "np">Introduction to Network Programming.
    <input type = "checkbox" name = "courses" value = "nd">Network Design or Network Simulation and Modeling.
    <br>
  </div>
  <br>

  <button name = "sig" id = "sig" style = "padding: 10px">Sign up</button>

</form>
</div>