I'm studying bootstrap, and in formulating the GRID system the col recognizes the values LG, MD, SM, but does not recognize the requested XS. When I resize the browser to mobile size it always puts the requested div in size 12 (full).
<!DOCTYPE html>
<html>
<head>
<title>Bootstrap Bill Turner</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link href="css/bootstrap.min.css" rel="stylesheet" media="screen">
<link href="css/estilo.css" rel="stylesheet" media="screen">
</head>
<style>
.c{
background-color:aquamarine;
border-right:1px solid black;
}
</style>
<body>
<div class="container">
<div class="row">
<div class="c col-lg-12 col-md-12 col-sm-6 col-xs-1">
esp
</div>
<div class="c col-lg-12 col-md-12 col-sm-6 col-xs-1">
esp
</div>
<div class="c col-lg-12 col-md-12 col-sm-6 col-xs-1">
esp
</div>
<div class="c col-lg-12 col-md-12 col-sm-6 col-xs-1">
esp
</div>
</div>
</div>
<script src="js/jquery-3.2.1.min.js"></script>
<script src="js/bootstrap.min.js"></script>
<script src="js/main.js"></script>
</body>
When resizing to mobile, it has the following layout:
Browser resized for mobile resolution
Someone to give me a light on what I'm doing wrong?
A print with the complete code, and the three forms of visualization: 
Full code and 3 forms of visualization As you can see, all right, except in COL XS. (I did not start LG because I did not have space in the same print)
