adding options to drop down box dynamically

I want to dynmically add some options to drop down box.but i want to mark one of them as selected .How can i do it?

DropdownBox = document.getElementById("xyzid");
 var optn = document.createElement("OPTION");
 optn.text="txt";
 optn.value="value";
 DropdownBox.options.add("values");

the above code is executed inside l开发者_JS百科oop with diff values .How can i mark one as selected?

DropdownBox.options[0].selected = true;

Change the index as needed.

Update:

HTML/JavaScript - Choose dropdown options using JavaScript