<html>
<head><title>mars test page</title></head>
<script>
function chk(obj)
{
var boxarray = document.getelementsbyname('c1');
for(var i=0;i<=boxarray.length-1;i++){
if(boxarray[i]==obj && obj.checked){
boxarray[i].checked = true;
}else{
boxarray[i].checked = false;
}
}
}
</script>
<body>
<form name="form1">
<input type="checkbox" name="c1" value="1" onclick="return chk(this);">a
<input type="checkbox" name="c1" value="2" onclick="return chk(this);">b
<input type="checkbox" name="c1" value="3" onclick="return chk(this);">c
</form>
</body>
</html>
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