两个select表单元素，怎样把第一个select的值传给第二个并做为第二个select表单的查询条件

<script><br>
var a1=Array();<br>
var a2=Array();<br>
a1[0]="javascript";<br>
a1[1]='css';<br>
a1[2]='php';<br>
a2[0]=Array(0,"javscript简介",0);<br>
a2[1]=Array(1,"javscript对象",0);<br>
a2[2]=Array(2,"javscript语法",0);<br>
a2[3]=Array(3,"javscript应用",0);<br>
a2[4]=Array(4,"css简介",1);<br>
a2[5]=Array(5,"css特点",1);<br>
a2[6]=Array(6,"css语法",1);<br>
a2[7]=Array(7,"css应用",1);<br>
a2[8]=Array(8,"php简介",2);<br>
a2[9]=Array(9,"php对象",2);<br>
a2[10]=Array(10,"php语法",2);<br>
a2[11]=Array(11,"php应用",2);<br>
function sel(val,obj,arr){<br>
obj.length=0;<br>
for(i=0;i<a2.length;i++){<br>
if(arr[i][2]==val)obj[obj.length]