Angular - unable to validate form of radio buttons

I have a form with only radio buttons, the form must be validated before submission (user must select something). After user selects a radio button some styling should apply using ngclass. I have two problems:
1. For some reason the last option is selected by default.
2. I am not able to get form validation attributes. The data is not refreshed when I select stuff on it.

plunkr

JS:

app.controller('MainCtrl', function($scope) {

  $scope.questionObject = [{
    "body": "abc"
  }, {
    "body": "def"
  }, {
    "body": "aghi"
  } ]
  $scope.selectAnswer=function(number,obj)
  {
    $scope.isChecked=number;
}
});

HTML:

  <div class="form-group" ng-form="form">
    <div class="col-lg-8 col-lg-offset-2 col-sm-12">
      <div class="row" ng-repeat="obj in questionObject track by $index">
        <div class="radio">
          <label class="choice" ng-class="{'choiceSelected': isChecked==$index}">
            <input type="radio"
                  name="optradio" 
                  ng-click="selectAnswer($index,questionObject.correct_answer)"
                  ng-model="radioInput">{{obj.body}}
          </label>
        </div>
      </div>
    </div>
  </div>
  <pre>
   dirty:{{form.$dirty}}
   pristine:{{form.$pristine}}
  </pre>

EDIT This is the form output. Never gets updated

   {
  "$error": {},
  "$name": "form",
  "$dirty": false,
  "$pristine": true,
  "$valid": true,
  "$invalid": false,
  "$submitted": false
}

Try like this. look for the ng-model and ng-required attributes for radio button.

<div name="myForm" novalidate ng-form>
    <div class="col-lg-8 col-lg-offset-2 col-sm-12">
      <div class="row" ng-repeat="obj in questionObject track by $index">
        <div class="radio">
          <label class="choice" ng-class="{'choiceSelected': isChecked==$index}">
        <input type="radio" name="optradio" ng-click="selectAnswer($index,questionObject.correct_answer)" ng-model="data.option" ng-value="obj.body" ng-required="data.option == ''">{{obj.body}}
      </label>

        </div>
      </div>
    </div>
  </div>

  <pre>
     dirty:{{myForm.$dirty}}
     pristine:{{myForm.$pristine}}
     $valid:{{myForm.$valid}}
     $invalid:{{myForm.$invalid}}
  </pre>

Plunker :https://plnkr.co/edit/crpV5QnSGdSv6rUBnTnR?p=preview