1. why the Jquery code in video3.1.js is not optimal?

2. What is Event-Delegation and why it is important?

3.  When will this code execute? and what does it do?
    $("button").on('click', function(e) {
      $(this).remove();
    }); 

4.  When will this code execute? and what does it do?
    $(".vacation").on("click", "button", function(e) {
        $(this).remove();

    });