1.添加option
  var theColor = document.getElementById("color");
  
  //var oOption1 = new Option("white","3");
  var oOption1 = document.createElement("OPTION");
  oOption1.innerHTML="white";
  oOption1.value ="3";
  theColor.appendChild(oOption1);

  var oOption2 = document.createElement("OPTION");
  oOption2.innerText = "black";
  oOption2.value = "4";
  theColor.options.add(oOption2);

[注]
   a.
     document.createElement
    document.createElement("OPTION"),document.createElement("tr"),
    document.createElement("table")...

    b.
      两种都可以使用oOptionX.selected = true来选择
      而第二个则还可以通过theColor.selectedIndex =2;
      或者theColor.value = "4"来选择
     
2.
  xSelect.children(i) == xSelect.options[i]
  xSelect.getElementsByTagName('option');

3.

<script>

	function showOptions(){
		var someNodeList = $('lstEmployees').getElementsByTagName('option');
		var nodes = $A(someNodeList);

		nodes.each(function(node){
				alert(node.nodeName + ': ' + node.innerHTML);
			});
	}
</script>

<select id="lstEmployees" size="10" >
	<option value="5">Buchanan, Steven</option>
	<option value="8">Callahan, Laura</option>
	<option value="1">Davolio, Nancy</option>
</select>

<input type="button" value="Show the options" onclick="showOptions();" >