小白一枚，求解释，二级联动菜单！！

<script type="text/javascript">
function pc(){
var province=["黑龙江","广东","江苏","甘肃"];
var ct=[["哈尔滨","大庆"],["广州","深圳"],["南京","苏州"],["兰州","酒泉"]];
var selected=document.getElementsById("province").value;
city.length=ct[selected-1].length;
for(i=0;i<ct[selected-1].length;i++){
city.options[i].value=i+1;
city.options[i].text=ct[selected-1][i];
}
}
</script>
\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
<select id="province" name="province" onchange="pc()">
<option value="1">黑龙江</option>
<option value="2">广东</option>
<option value="3">江苏</option>
<option value="4">甘肃</option>
</select>
<select name="city" id="city">
<option value="1" selected="selected">哈尔滨</option>
<option value="2">大庆</option>
</select>