问Knockoutjs单选按钮无法选择默认选中

EN

<input 
  type="radio" 
  name="uniqueQuestionName" 
  data-bind="checkedValue: $data,
    click: $root.setCorrectAnswer,
    checked: $root.correctAnswer" 
/>

function Question() {
  var self = this;
  this.name = "My Question";

  var i = 0;
  this.answers = ko.observableArray([
    new Answer(++i, "Answer 1", false),
    new Answer(++i, "Answer 2", true),
    new Answer(++i, "Answer 3", false)
  ]);

  this.setCorrectAnswer = function(correct) {
    if (correct !== self.correctAnswer()) {
      ko.utils.arrayForEach(self.answers(), function(answer) {
        answer.isRight(correct === answer);
      });

      self.correctAnswer(correct);
    }
    return true;
  };

  this.correctAnswer = ko.observable();

  this.correctAnswer.subscribe(function(newValue) {
    alert("The correct answer to " + this.name + " is now " + this.correctAnswer().name());
  }, this);
}


function Answer(id, name, isRight) {

  this.id = ko.observable(id);
  this.name = ko.observable(name);
  this.isRight = ko.observable(isRight);
}

var question = new Question();
//Set default correct answer just for example
question.correctAnswer(question.answers()[1]);
ko.applyBindings(question);

<script type="text/javascript" src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.4.0/knockout-min.js"></script>
<div data-bind="text:name"></div>
<div data-bind="foreach:answers">
  <label>
    <span data-bind="text: name"></span>
    <input type="radio" name="uniqueQuestionName" data-bind="checkedValue: $data,
        click: $root.setCorrectAnswer,
        checked: $root.correctAnswer" />
  </label>
  <br />
</div>

<hr/>

<pre data-bind="text: JSON.stringify(ko.toJS($root), null, 2)"></pre>