The select drop-down box of angularjs selects an option by default

The select drop-down box of angularjs selects an option by default. I
found a lot of links on the Internet, and it will not work after the experiment. For example common:

   <select  class="form-control"  ng-model="gender" >
            <option value="man" ng-selected="true">男性</option>
            <option value="woman">女性</option>
   </select>
有的把  ng-selected="true" 换成  ng-selected="1" 依然无效

effective method:

html:
 <select  class="form-control"    ng-model="vm.gender"
        ng-options="o.id as o.name for o in vm.genders"  >
 </select>
js中：
        genders:[
            {id:"man",name:"男性"},
            {id:"woman",name:"女性"}
        ],
        gender:  "man" 

Generate a default drop-down box for select