How to <if> this ?

[utw-Mephisto]

I have the following three drop downs :

<!-- Main -->
<td width="30%" class="alt2"><b>Game</b></td>
<td align="left" width="70%" class="alt1Active"> 
<SELECT NAME="Game">
<OPTION VALUE="Onslaught.ONSOnslaughtGame">Onslaught
<OPTION VALUE="XGame.xDeathMatch">Deathmatch
</SELECT> </td> </tr>
<!-- / Main -->

<!-- Deathmatch -->
<td width="30%" class="alt2"><b>Map</b></td>
<td align="left" width="70%" class="alt1Active">
<SELECT NAME="Map">
<OPTION VALUE="DM-Rankin">DM-Rankin
<OPTION VALUE="DM-Morpheus3">DM-Morpheus3
</SELECT> </td> </tr>
<!-- / Deathmatch -->

<!-- Onslaught -->
<td width="30%" class="alt2"><b>Map</b></td>
<td align="left" width="70%" class="alt1Active">
<SELECT NAME="Map">
<OPTION VALUE="ONS-Torlan">ONS-Torlan
<OPTION VALUE="ONS-Dria">ONS-Dria
</SELECT> </td> </tr>
<!-- / Onslaught -->

BUT: How can I use a <if> condition to show either the 2nd or the 3rd drop down depending on the first choice ?

Lets say someone chooes Onslaught in the first drop down

I would like to show the Onslaught drop down and the Deathmath not at all and the other way around