Bootstrap 单选按钮“已选中"旗帜

In case #1 works, in case #2 it do not works. Check the code bellow:

<div class="container">
    <div class="row">
        <h1>Radio Group #1</h1>
        <label class="radio-inline">
          <input name="radioGroup" id="radio1" value="option1" type="radio"> 1
        </label>
        <label class="radio-inline">
          <input name="radioGroup" id="radio2" value="option2" checked="checked" type="radio"> 2
        </label>
        <label class="radio-inline">
          <input name="radioGroup" id="radio3" value="option3" type="radio"> 3
        </label>
    </div>
    <div class="row">
        <h1>Radio Group #2</h1>
        <label for="year" class="control-label input-group">Year</label>
        <div class="btn-group" data-toggle="buttons">
            <label class="btn btn-default">
                <input name="year" value="2011" type="radio">2011
            </label>
            <label class="btn btn-default">
                <input name="year" value="2012" type="radio">2012
            </label>
            <label class="btn btn-default">
                <input name="year" value="2013" checked="checked" type="radio">2013
            </label>
        </div>  
    </div>  
</div>

您可以在这里看到它的实际效果:http://bootply.com/84165

<div class="row">
    <h1>Radio Group #2</h1>
    <label for="year" class="control-label input-group">Year</label>
    <div class="btn-group" data-toggle="buttons">
        <label class="btn btn-default">
            <input type="radio" name="year" value="2011">2011
        </label>
        <label class="btn btn-default">
            <input type="radio" name="year" value="2012">2012
        </label>
        <label class="btn btn-default active">
            <input type="radio" name="year" value="2013" checked="">2013
        </label>
    </div>
  </div>