#bg{
    width: 100%;
    height: auto;
}

.btn2{
    top: 77%;
}

 /* Large devices (desktops, 992px and up) */
 @media (min-width: 992px) { 
    #bg{
        height: 100vh;
    }

    .btn2{
        top: 77%;
    }
  
  }