Mixin Pattern using underscore, _.extend keyword

I want to make an example code based on 'Mixin Pattern'.

I have a code like below.

 define(["jquery","underscore", "backbone"], function($, _,Backbone)  {

//Mixin :
//
/* Sometimes you have the same functionality for multiple objects 
 *      and it doesn’t make sense to wrap your objects in a parent object.
 * For example, if you have two views that share methods but don’t 
 *      – and shouldn’t – have a shared parent view.
 */

//I can define an object that has attributes and methods that can be shared across different classes.
//This is called a mixin.

//Mixin Object
var C = Backbone.Model.extend({
    _c: function() {
        console.log("We are different but have C-Method shared");
    }
});

//To be Mixin-ed Object-1
var A = Backbone.Model.extend({
    a: 'A',
});

//To be Mixin-ed Object-2
var B = Backbone.Model.extend({
    b: 'B'
});

//underscore
_.extend(A.prototype, C);
_.extend(B.prototype, C);

return Backbone.Model.extend({
    initialize: function(){
        var testA = new A();
        testA.c();
        var testB = new B();
                    testA.c();

    }
});

});

If I run this code, there comes an error saying 'testA.c is not a function'. Judging by some example codes which I studied, This should work. Can you please let me know about the reason this code doesn't work as detail as possible?