How to give condition to dropdown on adding new row

i have a table with dropdown and other controls

am adding new row on drop down change using angular js

Customer	Pendings	Pending Value
		{{ReceiptsViewModel.PendingAmount}}

angular code to add new row

var ReceiptsApp = angular.module('ReceiptsApp', []);

ReceiptsApp.controller('ReceiptsController', function ($scope) {

$scope.rvm = [{}];

$scope.GetAccountBalance = function () {

$scope.rvm.push({

});

}

});

but my requirement is,

if i select dropdown am adding new row that is fine,

but am changing value first dropdown again,it is adding one more row.

but i want like if i select first dropdown add row, then add new row on selecting dropdown in second row

now dont add rows on change of first and 2nd row dropdown