Parse a XML element with AJAX

I need to parse a element from a XML file:

<a>
<b>45</b>
<c>67</c>
</a>

<a>
<b>98</b>
<c>89</c>
</a>

So I try to read and put in a html element the first value of b with the next code:

document.getElementById("apDiv3").innerHTML=xmlDoc.getElementsByTagName("b")[0].data;

or

document.getElementById("apDiv3").innerHTML=xmlDoc.getElementsByTagName("b")[0].value;

Code (markup):

and I get a undefined message.Where is the problem?

 

Try with this:

document.getElementById("apDiv3").innerHTML = xmlDoc.getElementsByTagName("b")[0].firstChild.nodeValue;

Code (markup):

 

yes it works,thanks, but now I have another problem : I have a radiogroup and I want to change a variable in function of what radio button of the radiogroup is selected