ASP.NET MVC Bootstrap form controls not aligned properly - asp.net-mvc

Question

In my following ASP.NET MVC Core project view, I want to display input, its corresponding label and submit button Add controls in the same line. but they are displaying in three separate lines, why?. How can I make them display in the same line (please note that the dropdown control is correctly showing its label and submit button Go in the same line). Note: Probably not related - but we know the content of the labels are coming from the data annotations of the corresponding model.
View:
<div class="col-md-9">
<form asp-controller="myControllerName" asp-action="myTestAction" method="post" class="form-horizontal">
<hr />
<div asp-validation-summary="All" class="text-danger"></div>
<div class="form-group">
<div class="col-md-9">
<label asp-for="SelectedYear" class="control-label"></label>
<select asp-for="SelectedYear" asp-items="Model.lstYears"></select><button type="submit" class="btn btn-info btn-xs">GO</button>
</div>
</div>
<div class="form-group">
<div class="col-md-9">
<label asp-for="testDesc" class="control-label"></label>
<input asp-for="testDesc" class="form-control"/><button type="submit" class="btn btn-info btn-xs">Add</button>
</div>
</div>
</form>
</div>
Snapshot of Page display from the above View
UPDATE:
After following a suggestion from the user #RatHat I've been able to display the three controls inline. But, still why they are not aligned left like the other three controls for dropdown. New display:

Answer 1

When using a .form-horizontal, you need to specify the number of columns that each element should cover:
<div class="form-group">
<!-- add the number of columns that the label should use -->
<label asp-for="testDesc" class="control-label col-sm-2"></label>
<!-- add the number of columns that the input should use -->
<div class="col-sm-8">
<input asp-for="testDesc" class="form-control">
</div>
<!-- add the number of columns that the Add button should use -->
<div class="col-sm-2">
<button type="submit" class="btn btn-info btn-xs">Add</button>
</div>
</div>