bootstrap 表单 input group 输入框内带字符 加文字 符号

                        <div class="form-group" id='rule4' style='display:none;'>
                            <div class="col-sm-12">
                                <div class="input-group">
                                    <span class="input-group-addon">请设置正确答案:</span>
                                    <input name='key' type="text" class="form-control" placeholder="默认是a" value='a'>
                                </div>
                                &nbsp;
                                <div class="input-group">
                                    <span class="input-group-addon">a</span>
                                    <input name='a' type="text" class="form-control" placeholder="必填，最多10个字">
                                </div>
                                &nbsp;
                                <div class="input-group">
                                    <span class="input-group-addon">b</span>
                                    <input name='b' type="text" class="form-control" placeholder="必填，最多10个字">
                                </div>
                            </div>
                        </div>