How to select defualt selected option?

<select ng-model="talkTime.operator" ng-change="updateMoileOperator(talkTime)">
			  <option value=""  selected="selected">-Select One-</option>
			  <option ng-repeat="x in mobileOperatorsNamesAndCodesNodes">{{x.mobileOperatorName}}</option>

I have use dropdrown list above system. when i use talkTime.operator=0; then it becomes bank. That means it is not selected on -Select One–,
When i click clear button ,dropdown list show default selected .That means it select -Select One–

How to do it?
Please help me…