Code:

<script type="text/javascript">
    function display(character) {
        var location = document.getElementById("secondItem");
        var content = document.getElementById("secondItem" + character);
        location.innerHTML = content.innerHTML;
    }
</script>

<form method="get" action="http://www.blabla.com/index.php">
  <input type="radio" name="firstItem" value="a" onclick="display('A')" /> a <br />
  <input type="radio" name="firstItem" value="b" onclick="display('B')" /> b <br />
  <input type="radio" name="firstItem" value="c" onclick="display('C')" /> c <br />
  <br /><br />
  <span id="secondItem"></span>
  <input type="submit" />
</form>

<span style="visibility: hidden" id="secondItemA">
  <input type="radio" name="secondItem" value="a1" /> a1 <br />
  <input type="radio" name="secondItem" value="a2" /> a2 <br />
  <input type="radio" name="secondItem" value="a3" /> a3 <br />
  <input type="radio" name="secondItem" value="a4" /> a4 <br />
</span>

<span style="visibility: hidden" id="secondItemB">
  <input type="radio" name="secondItem" value="b1" /> b1 <br />
  <input type="radio" name="secondItem" value="b2" /> b2 <br />
</span>

<span style="visibility: hidden" id="secondItemC">
  <input type="radio" name="secondItem" value="c1" /> c1 <br />
  <input type="radio" name="secondItem" value="c2" /> c2 <br />
  <input type="radio" name="secondItem" value="c3" /> c3 <br />
</span>