function MarqueeFun(demo1,demo2,demo3) {
    var demo1 = document.getElementById("'"+demo1+"'");
    var demo2 = document.getElementById("'" + demo2 + "'");
    var demo3 = document.getElementById("'" + demo3 + "'");
    var speed = 40;    //滚动速度值，值越大速度越慢
    //克隆demoC为demoB
    var str = "";
    for (var i = 0; i < 10; i++) {
        str += demo2.innerHTML;
    }
    //demoC.innerHTML = demoB.innerHTML + demoB.innerHTML + demoB.innerHTML + demoB.innerHTML + demoB.innerHTML + demoB.innerHTML
    demoC.innerHTML = str;
    if (demo3.offsetTop - demo1.scrollTop <= 0)    //当滚动至demoB与demoC交界时
        demo1.scrollTop -= demo2.offsetHeight    //demoA跳到最顶端 
    else {
        demo1.scrollTop++
    } 
}