<script type="text/javascript">
function check_dd() {
    if(document.getElementById('select_branch').value == "") {
        document.getElementById('Branch1').style.display = 'none';
        document.getElementById('Branch2').style.display = 'none';
    }
    if(document.getElementById('select_branch').value == "1") {
        document.getElementById('Branch2').style.display = 'none';
    } else {
        document.getElementById('Branch2').style.display = 'block';
    }
    if(document.getElementById('select_branch').value == "2") {
        document.getElementById('Branch1').style.display = 'none';
    } else {
        document.getElementById('Branch1').style.display = 'block';
    }
    if(document.getElementById('select_branch').value == "3") {
        document.getElementById('Branch1').style.display = 'none';
    } else {
        document.getElementById('Branch1').style.display = 'block';
    }
}
</script>
<select class="default" id="select_branch" name="select_branch" onchange="check_dd();">
    <option value="" selected>Select question...</option>
    <option value="1">Question One</option>
    <option value="2">Question Two</option>
    <option value="2">Question three</option>
</select>

<div id="Branch1" style="display:none;">one</div>
<div id="Branch2" style="display:none;">two</div>
<div id="Branch3" style="display:none;">three</div>