.us_top{
    height: 640px;
    background-image: url("../images/backimg.jpg");
}
.us_content{
}
.saaa{
    margin-top: 100px;
}
.img{
    height: 640px;
    line-height: 640px;
}
.img img{
    max-width: 100%;
    height: auto;
}