<select name="big" multiple>
<option value="1">a</option>
<option value="3">b</option>
<option value="5">c</option>
</select>
<script>
function Sel()
{
document.getElementById("big")[0].selected=true
document.getElementById("big")[1].selected=true
}
</script>
<input type=button onclick=Sel()>
<option value="1">a</option>
<option value="3">b</option>
<option value="5">c</option>
</select>
<script>
function Sel()
{
document.getElementById("big")[0].selected=true
document.getElementById("big")[1].selected=true
}
</script>
<input type=button onclick=Sel()>
<option value="1">a</option>
<option value="3">b</option>
<option value="5">c</option>
</select>
<script>
function Sel()
{
for(i=0;i<document.getElementById("big").options.length;i++)
{
if(document.getElementById("big").options[i].value=="3" || document.getElementById("big").options[i].value=="5")
document.getElementById("big").options[i].selected=true
}
}
</script>
<input type=button onclick=Sel()>