two select elements exchange single option

W3C ver.
var s1=document.getElementById('s1');
var s2=document.getElementById('s2');

function AddDelOption(a,b) {
  var txt=a.options[a.selectedIndex].value;
  b.options.add(new Option(txt,txt));
  a.remove(a.selectedIndex);
}
document.getElementById('addBtn').addEventListener('click',function(){AddDelOption(s1,s2);},false);
document.getElementById('delBtn').addEventListener('click',function(){AddDelOption(s2,s1);},false);

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jQuery ver. var s1=document.getElementById('s1');
var s2=document.getElementById('s2');

function AddDelOption(a,b) {
  var txt=$(a).val();
  $(b).append("<option value=\"" + txt + "\">" + txt + "</option>");
  a.remove(a.selectedIndex);
}

$('#addBtn').click(function(){AddDelOption(s1,s2);});
$('#delBtn').click(function(){AddDelOption(s2,s1);});