AngularJS下拉列表select在option动态变化之后多出了一个错误项的问题

　　<div ng-app="TestApp">
        <div ng-controller="TestAppCtrl">
            <label>options变化之后会出错：</label>
            <select ng-model="selectedSite">
                <option  value="" >---请选择---</option>
                <option ng-repeat="site in sites" value="{{site.url}}">{{site.name}}</option>
            </select>
            <input type="button" ng-click="reload()" value="更新">
        </div>
    <div>

        var testApp = angular.module(‘TestApp‘, []);
        testApp.controller(‘TestAppCtrl‘, [‘$scope‘, 
        function($scope) {
            var sites=[{"url":"http://www.baidu.com","name":"百度"},
                       {"url":"http://www.google.com","name":"谷歌"},
                       {"url":"http://www.yahoo.com","name":"雅虎"},
                        ]
            var sites2=[{"url":"http://www.gmail.com","name":"GMAIL"},
                       {"url":"http://www.abc.com","name":"ABC"},
                       {"url":"http://www.xyz.com","name":"XYZ"},
                        ]
                        
            var Init = function(){
                $scope.sites=sites;
            }
            
            var Reload = function(){
                $scope.sites=sites2;
            }
　　　　　　　//加载页面之后初始化
            Init();
　　　　　　　//注册更新按钮的点击事件，点击之后更新select的option列表
            $scope.reload=Reload;
            }]
        );

    <div ng-app="TestApp">
        <div ng-controller="CorrectedAppCtrl">
            <label>正确写法：</label>
            <select ng-model="selectedSite" ng-options="site as site.name for site in sites track by site.url">
                <option  value="" >---请选择---</option>
            </select>
            <input type="button" ng-click="reload()" value="更新">
        </div>
    <div>