JavaScript判断表单提交时哪个radio按钮被选中的方

<script type="text/javascript">
function findButton() {
var myForm = document.forms.animalForm;
var i;
for(i=0;i<myForm.marsupial.length; i++) {
  if(myForm.marsupial[i].checked) {
   break;
  }
}
alert("You selected \""+myForm.marsupial[i].value+"\".");
}
</script>
<form name="animalForm">
<input type="radio" name="marsupial" value="kangaroo" />jb51.
<br />
<input type="radio" name="marsupial" value="Opossum" />Opossum
<br />
<input type="radio" name="marsupial" value="Tasmanian Tiger" />Tasmanian Tiger
<br />
<input type="button" name="GO" value="GO" onclick="findButton()" />
</form>