JSON.Parse Returns Undefined

I'm working on a poll system. After the user submits their poll answer choice, it should return JSON with all answers so I can display them.

After submitting the AJAX form, it returns the JSON correctly like this:

[{"answer_1":0,"answer_2":1,"answer_3":0,"answer_4":0}]

But when I try to parse it, all answers return undefined.

This is how I parse it:

    $("#poll-form").submit(function(event) {
        var data = $("#poll-form").serialize();
        $.ajax({
            url: 'ajax.php',
            type: 'POST',
            data: data,
            success: function(response) {
                var res = JSON.parse(response);

                $(".poll-content").html("<h1>Answer:</h1>" + res.answer_1); // res.answer_1 returns undefined
            }
        });

        event.preventDefault();
    });

What am I doing wrong? Why is it returning undefined? All suggestions are welcome.