如何更改为默认的所选下拉列表

Question

我在HTML上创建了三个下拉菜单，并且制作了一个JavaScript程序来提醒您有重复的选定值。如果在警报后选定的下拉列表在警报后返回其默认值，该怎么办。

function hey() {

  var count = document.getElementsByClassName('data').length;
  //var cars = document.getElementsByClassName('data')[0].value;
  var arr = [];
  var text = "";
  var i = 0;
  while (i < count) {
    if ((document.getElementsByClassName('data')[i].value) == "") {} else {
      arr.push(document.getElementsByClassName('data')[i].value);
    }
    i++;
  }
  document.getElementById("demo").innerHTML = arr;
  var uniqueval = arr.filter(function(itm, i, arr) {
    return i == arr.indexOf(itm);
  });
  if (arr.length > uniqueval.length) {
    alert("Has duplicate")
  }
}

<div class="info">
  <select name="type" class="data" onchange="hey()">
    <option value="">---select---</option>
    <option value="teacher">Teacher</option>
    <option value="student">Student</option>
    <option value="students">Students</option>
  </select>
  <select name="class" class="data" onchange="hey()">
    <option value="">---select---</option>
    <option value="teacher">Teacher</option>
    <option value="student">Student</option>
    <option value="students">Students</option>
  </select>
  <select name="class" class="data" onchange="hey()">
    <option value="">---select---</option>
    <option value="teacher">Teacher</option>
    <option value="student">Student</option>
    <option value="students">Students</option>
  </select>
  <p id="demo"></p>
</div>
<p>When I select another dropdown and its been already selected <br> 
Theres have an alert message and when I click OK I want to return its default value which is --select--</p>

我要在选定下拉菜单的警报后返回--select--值。