根据代码，请问四个 div 标签从上到下的排列顺序（以 class 值代指）是（）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
div {
width: 20px;
height: 20px;
}
.item1 {
z-index: -10;
background-color: yellow;
}
.item2 {
position: fixed;
z-index: 1;
background-color: red;
margin-top: 10px;
}
.item3 {
position: fixed;
z-index: -1;
background-color: green;
margin-top: 40px;
}
.son {
position: fixed;
z-index: 100;
background-color: black;
margin-top: 30px;
}
</style>
</head>
<body>
<div class="item1"></div>
<div class="item2"></div>
<div class="item3">
<div class="son"></div>
</div>
</body>
</html> 
