Javaskript - skrývání

<script type="text/javascript">
function hideNo(obj, a){
    var e=document.getElementById(a);
    if(!e)return true;
        e.style.display="block"
    return true;
}
         
function hideYes(obj, a){
    var e=document.getElementById(a);
    if(!e)return true;
        e.style.display="none"
    return true;
}
</script>

<select name="bla">
    <option value="a" onclick="return hideNo(this, 'a'); return hideYes(this, 'b')">A</option>
    <option value="b" onclick="return hideNo(this, 'b'); return hideYes(this, 'a')">B</option>
</select>

<span id="a" style="display: none">
    A
    <br>
</span>

<span id="b" style="display: none">
    B
    <br>
</span>