i did a scaffold to make my DB and menus for group i need a drop down box instead of a text field

here is the code i have atm

```
    <%= form_for(@ad) do |f| %>
  <% if @ad.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@ad.errors.count, "error") %> prohibited this ad from being saved:</h2>

      <ul>
      <% @ad.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :ad_name %><br />
    <%= f.text_field :ad_name %>
  </div>
  <div class="field">
    <%= f.label :group %><br />
    <%= f.text_field :group %>
  </div>
  <div class="field">
    <%= f.label :description %><br />
    <%= f.text_area :description %>
  </div>
  <div class="field">
    <%= f.label :credits %><br />
    <%= f.text_field :credits %>
  </div>
  <div class="field">
    <%= f.label :image_url %><br />
    <%= f.text_field :image_url %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

```

if you can let me know how to change group to a pull down with selections it would be helpful.