动画

&:nth-child(3){
               width: 29*$r;
              height: 504*$r;
              background: url('./image/index/3.png') no-repeat center;
              background-size: cover;
              right: 340*$r;
              bottom: 130*$r;
              opacity: 0;
              transition: opacity .7s ease-in 3.4s;
              &.on {
                opacity: 1;
              }
            }