Bootstrap下拉列表水平对齐

Question

我有三个下拉列表，其中包括从用户获得生日的天数和年数 我不想把它们放在桌子里。我正在尝试使用Div元素来提高效率。

如何根据上述文本框修复它们？

以下代码适用于我的注册表格

        <div class="text-center">
            or
        </div>             

            <label>Firstname</label>
            <div>
             <input type="text" class="form-control" placeholder="Username">                  
            </div>

            <label>Lastname</label>
            <div>
                <input type="text" class="form-control" placeholder="Lastname">  
            </div>

            <label>Email</label>
            <div>
                <input type="text" class="form-control" placeholder="Email">  
            </div>

            <label>Password</label>
            <div>
                <input type="text" class="form-control" placeholder="Password">  
            </div>

            <label>Gender</label>
            <div>
                <select class="form-control">
                <option value="Male">Male</option>
                <option value="Female">Female</option>              
                </select>
            </div><br />

           <div class="text-center">
                <label>Birthday</label>
                <div class="form-inline" role="form">

                    <div class="form-group">
                        <div class="input-group">
                            <select class="form-control" placeholder="Day">                           
                            <option value="1">1</option>
                            <option value="2">2</option>            
                            </select>
                        </div>
                    </div>

                    <div class="form-group">
                        <div class="input-group">
                          <div class="input-group">
                            <select class="form-control" placeholder="Month">                           
                            <option value="1">January</option>
                            <option value="2">February</option>             
                            </select>
                        </div>

                        </div>
                    </div>

                    <div class="form-group">
                         <div class="input-group">
                            <select class="form-control" placeholder="Year">                           
                            <option value="1950">1950</option>
                            <option value="1951">1951</option>              
                            </select>
                        </div>
                    </div>

                </div> <br />
            </div>
            <div class="text-center">
                <input class="btn btn-success" type="submit" value="Register" />
            </div>

    </div>