angularjs select设置默认值

<select ng-change="changeRate(select_rate)" ng-model="select_rate" ng-options="k as v for (k, v) in rate_obj">
</select>

$scope.rate_obj = {
    8:'8倍速',
    4:'4倍速',
    2:'2倍速',
    1:'1倍速',
};
$scope.select_rate = '1'; //下拉框

重点：不能手动写option,要数据渲染