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>