Unity 5 C# - onClick.AddListener in a foreach loop using lambda expressions

I'm trying to dynamically create Unity 5 UI buttons that each set a global variable decision_id to a different value when clicked.

The problem I'm having with this implementation is that they all set decision_id to the exact same value right now. That value being the last one in my foreach loop.

Here's my code:

IEnumerator make_decision(XmlNode options_node) {
    // reset decision_id
    decision_id = "nothing";

    // spawn a button for each option_node in options_node
    int i = 1;
    var new_option_1 = Instantiate(option_1);
    var new_option_2 = Instantiate(option_2);
    var new_option_3 = Instantiate(option_3);
    var new_option_4 = Instantiate(option_4);
    foreach (XmlNode option_node in options_node) {
        XmlNode temp_option_node = option_node;
        string option_id = temp_option_node.Attributes["id"].Value;
        Debug.Log(option_id);
        switch (i) {
            case 1:
                new_option_1.GetComponentsInChildren<Text>()[0].text = option_node.InnerText;
                new_option_1.GetComponent<Button>().onClick.AddListener(() => set_decision_id(option_id));
                new_option_1.transform.SetParent(canvas.transform, false);
                break;
            case 2:
                new_option_2.GetComponentsInChildren<Text>()[0].text = option_node.InnerText;
                new_option_2.GetComponent<Button>().onClick.AddListener(() => set_decision_id(option_id));
                new_option_2.transform.SetParent(canvas.transform, false);
                break;
            case 3:
                new_option_3.GetComponentsInChildren<Text>()[0].text = option_node.InnerText;
                new_option_3.GetComponent<Button>().onClick.AddListener(() => set_decision_id(option_id));
                new_option_3.transform.SetParent(canvas.transform, false);
                break;
            case 4:
                new_option_4.GetComponentsInChildren<Text>()[0].text = option_node.InnerText;
                new_option_4.GetComponent<Button>().onClick.AddListener(() => set_decision_id(option_id));
                new_option_4.transform.SetParent(canvas.transform, false);
                break;
            default:
                Debug.Log("Too many option nodes! Should only be four per options section");
                break;
        }
        i++;
    }

    while (decision_id == "nothing") {
        yield return new WaitForSeconds(.5f);
        // the button will change decision_id for us eventually
        // decision_id = "monarchy";
    }

    // despawn each button spawned above
    Destroy(new_option_1);
    Destroy(new_option_2);
    Destroy(new_option_3);
    Destroy(new_option_4);

}

I've searched this issue and have found lots of talk about the lambda's capturing instances instead of values... but I've already gone out of my way to prevent this by capturing both the XmlNode itself AND the id string in local variables.

Is there something super obvious that I'm missing here?