CSS中图片居中显示不拉伸，且占满盒子大小

CSS中图片居中显示不拉伸，且占满盒子大小

<div class="img_box">
	<img class="img" src='URL'/>
div>

.img_box{
	width:200px;
	height:200px;
	position:relative;
}
.img {
    max-width: 100%;
    max-height: 100%;
    position: absolute;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    margin: auto;
    display: block;
}