Setting Selected With CreateElement("option")

I´m trying to create a dropdown menu through createElement, browser is IE.
The problem is setting one of the options as "selected".

So far I have:
options = editor.config[txt];
el = document.createElement("select");

for (var i in options) {
var op = document.createElement("option");
op.appendChild(document.createTextNode(i));
op.value = options[i];
el.appendChild(op);
}

I get my dropdown menu, but how do I set selected?
I´ve tried
op.selected = true
without success.