下拉列表css无法正确显示

Question

我正在使用bootstrap显示下拉列表。问题是样式没有应用于下拉列表。

当我查看它显示的源代码时......

<div class="form-group">
    <label class="control-label col-md-2" for="State">State</label>
    <div class="col-md-10">
        <select htmlAttributes="{ class = form-control }" id="State" name="State"><option 
    value="CT">CT</option>

为什么类形式控件没有被添加到select标签？

<div class="form-group">
    @Html.LabelFor(m => m.State, htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownListFor(m => m.State, db.StateDiscountRates.Select(c => new SelectListItem { Value = c.State, Text = c.State }).ToList().OrderBy(c => c.Text)
                    .GroupBy(test => test.Text)
                          .Select(group => group.First()), new { htmlAttributes = new { @class = "form-control" } })
        @Html.ValidationMessageFor(model => model.Address, "", new { @class = "text-danger" })
    </div>
</div>