io ho questa select:
<select name="Sportelli" style="width:155px;" size="5">
<option value="-1" selected>-- Seleziona Sportello --</option>
<option value="0">Campodarsego</option>
<option value="1">Camposampiero Unione</option>
<option value="2">Cittadella</option>
<option value="3">Limena</option>
<option value="4">Montegrotto Terme</option>
<option value="5">Piombino Dese</option>
<option value="6">Rubano</option>
<option value="7">San Martino di Lupari</option>
<option value="8">Torreglia</option>
<option value="9">Trebaseleghe</option>
<option value="10">Vigodarzere</option>
<option value="11">Vigonza</option>
</select>

vorrei che solo quando scelgo Camposampiero Unione si visualizzasse la select:
<select name="Sportelli" style="width:155px;" size="5">
<option value="-1" selected>-- Seleziona Sportello --</option>
<option value="0">Campodarsego</option>
<option value="1">Camposampiero Unione</option>
<option value="2">Cittadella</option>
<option value="3">Limena</option>
<option value="4">Montegrotto Terme</option>
<option value="5">Piombino Dese</option>
<option value="6">Rubano</option>
<option value="7">San Martino di Lupari</option>
<option value="8">Torreglia</option>
<option value="9">Trebaseleghe</option>
<option value="10">Vigodarzere</option>
<option value="11">Vigonza</option>
</select>
<div id="divSelnascosta" style="display:none;">
<select name="comuni_unione">
<option>Villanova</option>
<option>Camposampiero</option>
</select>
</div>

come posso fare?