Using select lists with radio buttons

I have a form that has 5 radio buttons. Two of those have optional additional parameters that the user can select. If radio 1 is selected, then only the optional items for that radio would make sense to be populated. Is there a way using the Forms API to accomplish this?

I have the following code. The Optionals Extra2 is the olnly select boxes that are shown. If I change the $form['mc']['cats'] to $form['mc']['cats2'] I can get it to display but it's not in the same fieldset as the rest.

$form['mc'] = array(
    '#type' => 'fieldset',
    '#title' => t('Membership Categories (select only one)'),
    );




$form['mc']['cats'] = array(
    '#type' => 'fieldset',
    '#title' => FALSE,
    '#prefix' => '<div style="background-color:red;">',
    '#suffix' => '</div>',
    );

$form['mc']['opts'] = array(
    '#type' => 'fieldset',
    '#title' => FALSE,
    );

$membership_cats = array(
    0 => t('Option 1'),
    1 => t('Option 2'),
    2 => t('Option 3'),
    3 => t('Option 4'),
    4 => t('Option 5'));


    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#prefix'] = '<div style="background-color:salmon;float:left;width:50%;">';
    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#suffix'] = '</div>';

    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#type'] = 'radios';
    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#title'] = t('Membership Category');
    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#description'] = t('Please select your membership category.');
    $form['mc']['cats']['profile_memberlist']['field_membership_category']['#options'] = $membership_cats;


$optionals = array(
    0 => t('No Thanks'),
    1 => t('Optional 1'),
    2 => t('Optional 2'));
$optionals2 = array(
    0 => t('No Thanks'),
    1 => t('Optional 3'),
    2 => t('Optional 4'));


$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#type'] = 'select';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#prefix'] = '<div style="background-color:lime;">';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#suffix'] = '</div>';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#title'] = t('Optional Extras');
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#options'] = $optionals;

$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#type'] = 'select';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#prefix'] = '<div style="background-color:lime;">';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#suffix'] = '</div>';
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#title'] = t('Optional Extras2');
$form['mc']['cats']['profile_memberlist']['field_optional_extras']['#options'] = $optionals2;