If checkbox = checked show dropdown menu

JavaScript
2 
<label for="platypus">Are you a platypus?</label>
<input id="platypus" type="checkbox" name="monotreme" value="platypus" />
  <select name="answer" id="answer">
    <option value="yes">Yes</option>
    <option value="no">No</option>
  </select>

javascript to put in the <head>:

window.onload = function() {
  var c = document.getElementById('platypus')
  c.onchange = function() {
    if (c.checked == true) {document.getElementById('answer').style.display = 'inline';}
    else {document.getElementById('answer').style.display = '';
    }
  }
}

CSS:

#answer {display:none;}

The dropdown will go away when you unselect the checkbox.