Contingent selects - AJAX style

If I have an html.erb file, wherein one select box's selections
displayed are contingent upon the value chosen in another select box
(for example, if I have a state select box, contingent on the country
showm in the country select box, so that if I have United States
chosen as the country, and I switch to Canada, only the 12 provinces
of Canada should now be available in the state select box) how can I
accomodate that in an AJAX-manner? Below is the html.erb code I am
trying to get this into (note I am using the Carmen plugin) Thanks,
Janna B.

<p>
     <%= f.label :state %><br />
  <%= f.state_select :state %>
  </p>
  <p>
      <%= f.label :country %><br />
      <%= f.country_select :country, 'US' %>
   </p>