Use two CSS classes in C # Razor MVC [closed]

I'm wanting to use two CSS classes in Razor. Is it possible to do that? Because I can not.

I'm doing the following:

    <div class="form-group">
        @Html.LabelFor(model => model.ativo, htmlAttributes: new { @class = "control-label col-md-2", @id = "ativo" })
        <div class="col-md-10">
           @Html.CheckBoxFor(model => model.ativo, new { htmlAttributes = new { @class = "form-control flat" } })
            @Html.ValidationMessageFor(model => model.ativo, "", new { @class = "text-danger" })
        </div>
    </div>

But the stylization of the "flat" class does not work. Does anyone have a solution?