在angularjs中使用select 赋值 ng-options配置方法该怎么做？

$scope.years = [2014, 2015, 2016];

 <select ng-model="item" ng-options="item as y for y in years">
 </select>

$scope.item = 2016;
$scope.years = [2014, 2015, 2016];

$scope.sites = [
 {site : "baidu", url : "https://www.baidu.com"},
 {site : "163", url : "http://www.163.com"},
 {site : "sina", url : "http://www.sina.com"}
];

 <select ng-model="s.site" ng-options="s.site as s.site group by site.group for s in sites">
 </select>

$scope.site = "163";
$scope.sites = [
 {site : "baidu", url : "https://www.baidu.com"},
 {site : "163", url : "http://www.163.com"},
 {site : "sina", url : "http://www.sina.com"}
];

$scope.cars = {
car1 : {brand : "BYD", model : "Y50", color : "red"},
car2 : {brand : "CC", model : "HF", color : "white"},
car3 : {brand : "JL", model : "JL10D", color : "black"}
};

<select ng-model="myCar" ng-options="y.brand for (x, y) in cars">
</select>

$scope.site = "BYD";
$scope.cars = {
car1 : {brand : "BYD", model : "Y50", color : "red"},
car2 : {brand : "CC", model : "HF", color : "white"},
car3 : {brand : "JL", model : "JL10D", color : "black"}
};

label for value in array
select as label for value in array
label group by group for value in array
select as label group by group for value in array track by trackexpr

label for (key , value) in object
select as label for (key , value) in object
label group by group for (key, value) in object
select as label group by group for (key, value) in ob