我是新手，试图提高我的技能，我只是卡住了，我有6个盒子，3个左侧和3个右侧，我的问题是为什么盒子号码'4'和'5'之间有空格，当我'在'sm，md，lg'尺寸上？如何在没有空格的情况下将第5号箱子放在第4号箱子下面？在那里带走那些“白色空间”并仍然使这个工作像这样？
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
<style>
.box{
width:30%;
height:250px;
border: solid black 1px;
}
.boxx{
width:100px;
height:100px;
border: solid white 1px;
}
.bo{
width:50%;
height:550px;
border: solid black 1px;
}
* {
box-sizing: border-box;
box-sizing: border-box;
-webkit-box-sizing -moz-box-sizing: border-box;
}
</style>
</head>
<body>
<body>
<div class="row">
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="boxx mx-auto">
white space
</div>
</div>
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="box mx-auto">
2
</div>
</div>
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="bo mx-auto">
3
</div>
</div>
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="box mx-auto">
4
</div>
</div>
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="boxx mx-auto">
white space
</div>
</div>
<div class="col-lg-6 col-md-6 col-sm-6 col-xs-12">
<div class="box mx-auto">
5
</div>
</div>
</div>
</body>
</html>
</body>
</html><!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
<style>
.box{
width:30%;
height:250px;
border: solid black 1px;
}
.bo{
width:50%;
height:550px;
border: solid black 1px;
}
* {
box-sizing: border-box;
box-sizing: border-box;
-webkit-box-sizing -moz-box-sizing: border-box;
}
</style>
</head>
<body>
<div class="row">
<div class="col-md-6">
</div>
<div class="col-md-6">
<div class="box mx-auto">
2
</div>
</div>
<div class="col-md-6">
<div class="bo mx-auto">
3
</div>
</div>
<div class="col-md-6">
<div class="box mx-auto">
4
</div>
<div class="box mx-auto">
5
</div>
</div>
</div>
</body>
</html>