Sending the selected option of a select via ajax


i want to create a form which contains 3 select-boxes which content
depend on each other.
i created the first select box:

<select id="timeblock_project" name="timeblock[project]" onclick="<%=
remote function(:update => 'new_sub_project', :url => { :action =>
'timeblock_submenu_tag', :id => ???}) -%> ">
  <% for project in @projects %>
    <option value="<>">
  <% end %>

<td id="new_sub_project">

the :id => ??? should be replaced with the id of the project in the
selected option-tag, but.. how?