css图片高度固定，宽度自适应

Question

如何让css图片高度固定，宽度自适应

Answer 1

img {
    display:block;
    height:100px;
    margin:0 auto;/*图片居中*/
}

Answer 2

img { height: 100% }

Answer 3

// 1
img {
  heigth:100%;
  width:auto;
}
// 2
box{
  background-image:url(''),
  background-size: auto 100%;
  background-repeat: no-repeat;
  background-position: center;
}