jQuery Set Select Index

I have an select box:

<select id="selectBox">
  <option value="0">Number 0</option>
  <option value="1">Number 1</option>
  <option value="2">Number 2</option>
  <option value="3">Number 3</option>
  <option value="4">Number 4</option>
  <option value="5">Number 5</option>
  <option value="6">Number 6</option>
  <option value="7">Number 7</option>
</select>

I’d like to set one of the options as “selected” based on it’s selected index.

For example, if I am trying to set “Number 3”, I’m trying this:

$('#selectBox')[3].attr('selected', 'selected');

But this doesn’t work. How can I set an option as selected based on it’s index using jQuery?

Thanks!