把一个下拉框中的选项添加到另一个中

<html>
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<body>
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<form name="aa">
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<table align="center">
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<tr>
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<td>
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<select name="xuanze" size="6" style="80px">
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<option value="海淀">海淀</option>
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<option value="朝阳">朝阳</option>
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<option value="东城">东城</option>
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<option value="西城">西城</option>
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<option value="宣武">宣武</option>
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<option value="丰台">丰台</option>
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</select>
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</td>
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<td align="center">
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<input type="button" value="添加>>" onclick="javascript:add()"/>
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<br>
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<input type="button" value="<<删除" onclick="javascript:del()"/>
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</td>
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<td>
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<select name="xuanzhong" size="6" style="80px"></select>
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</td>
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</tr>
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</table>
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</form>
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</body>
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function isexist(va){
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var ee = false;
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for(var i=0;i<document.aa.xuanzhong.options.length;i++)
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if(document.aa.xuanzhong.options[i].value==va)
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{ ee=true; break;}
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return ee;
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}
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function add(){//添加
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for(var i=0;i<document.aa.xuanze.options.length;i++)
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if(document.aa.xuanze.options[i].selected) {
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if(!isexist(document.aa.xuanze.options[i].value)) //是否已经添加？？
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document.aa.xuanzhong.options[document.aa.xuanzhong.options.length]
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= new Option(document.aa.xuanze.options[i].value,document.aa.xuanze.options[i].text);
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}
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}
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function del(){
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for(var i=document.aa.xuanzhong.options.length-1;i>=0;i--)
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if(document.aa.xuanzhong.options[i].selected)
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document.aa.xuanzhong.options[i] = null; //删除一个项目
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}
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</script>
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</html>