function show(txt){

     document.getElementById('top1').style.display="none";
     document.getElementById('top2').style.display="none";
     document.getElementById('top3').style.display="none";
     document.getElementById('top4').style.display="none";
     document.getElementById('top5').style.display="none";
     document.getElementById('top6').style.display="none";
with(txt){
     document.getElementById(txt).style.display="block";
  }
}