Custom script disabling radio buttons

function disable_Options(c) {

// Get the list of radio options with class name form-radio
var radio = document.getElementsByClassName("form-radio");

// Check each class name radio button
    for(var i = 0; i < radio.length; i++) {


      //Set what specific conditions if an option is selected and list the ones that will be disabled.
      // radio[0] is selected then disable radio[1]
        if(radio[0].checked) {

          radio[1].disabled = true;
          radio[1].parentNode.style.opacity = "0.7";


        }

      // if radio [2] is selected then disable radio[3]
        else if(radio[2].checked) {
          radio[3].disabled = true;
          radio[3].parentNode.style.opacity = "0.7";

        }


        radio[i].disabled = false;
        radio[i].parentNode.style.opacity = "1";


    }


}