关于ListBox，这个代码怎么写

Question

<select id=s1 onchange="x()">
<option>5
<option>10
<option>15
</select>
<select id=s2></select>
<script>
function x()
{
document.getElementById("s2").options[0] = new Option("1","1")
for(i=1;i<4;i++)
document.getElementById("s2").options[i] = new Option(i*parseInt(document.getElementById("s1").options[document.getElementById("s1").selectedIndex].text),i*parseInt(document.getElementById("s1").options[document.getElementById("s1").selectedIndex].text))
}
</script>

Answer 1

<select id=s1 onchange="x()">
<option>
<option>5
<option>10
<option>15
</select>