how to use jquery selectbox plugin for five element at the same page?

how to use jquery selectbox plugin for five element at the same page?

I have 3 select box at the same page, and i wanna use jquery selectbox
plugin for all 3 selectbox.
The problem is that plugin works only the first element ... and the rest
remain normal.
someone know why is that?
my code look like this:
<select class="ex">
<option>test</option>
<option>test</option>
<option>test</option>
<option>test</option>
</select>
<select class="ex">
<option>test</option>
<option>test</option>
<option>test</option>
<option>test</option>
</select>
<select class="ex">
<option>test</option>
<option>test</option>
<option>test</option>
<option>test</option>
</select>
and my js code look like this:
$(".ex").selectbox();