AngularJS Select绑定的是对象：
$scope.cars = {
car01 : {brand : "Ford", model : "Mustang", color : "red"},
car02 : {brand : "Fiat", model : "500", color : "white"},
car03 : {brand : "Volvo", model : "XC90", color : "black"}
};<select ng-model="selectedCar" ng-options="y.brand for (x, y) in cars"></select>现在想给select一个默认值怎么设置呢？
这样就是默认选中第一项
<option>--please select--</option>