Javascript动态生成下拉列表

用Javascript实现根据下拉列表category的选择结果，改变下拉列表sub_category的选项
<mce:script language="javascript" type="text/javascript">
<!--
function changeSubCategory() {
var sel_category = document.getElementById("category");
var sel_sub_category = document.getElementById("sub_category");
var len = sel_sub_category.length
for ( var i = 0; i < len; i++) {
sel_sub_category.remove(sel_sub_category.length - 1);
//清除原有的option
}
if (sel_category.value == "本科") {
var opt1 = document.createElement("option")
opt1.text = opt1.value = "本科一年级"
sel_sub_category.add(opt1)
var opt2 = document.createElement("option")
opt2.text = opt2.value = "本科二年级"
sel_sub_category.add(opt2)
var opt3 = document.createElement("option")
opt3.text = opt3.value = "本科三年级"
sel_sub_category.add(opt3)
var opt4 = document.createElement("option")
opt4.text = opt4.value = "本科四年级"
sel_sub_category.add(opt4)
} else if (sel_category.value == "硕士") {
var opt1 = document.createElement("option")
opt1.text = opt1.value = "硕士一年级"
sel_sub_category.add(opt1)
var opt2 = document.createElement("option")
opt2.text = opt2.value = "硕士二年级"
sel_sub_category.add(opt2)
} else if (sel_category.value == "博士") {
var opt1 = document.createElement("option")
opt1.text = opt1.value = "博士一年级"
sel_sub_category.add(opt1)
var opt2 = document.createElement("option")
opt2.text = opt2.value = "博士二年级"
sel_sub_category.add(opt2)
}
}
// --></mce:script>