Call for after onclick event occured

Hi

This is the code

$form['option_city'] = array(
'#type' => 'radios',
'#title' => t('City'),
'#description' => t('Reservation'),
'#options' => array(
t('City'),
t('Address'),
),
'#default_value' => variable_get('option_city', 0) // default to Never

);

The code display the two radio button ,City and Address,

What i want here ?

When user click/Select the City , Then i want display the City form,

When user click/Select the Address, Then i want display the Address form,

so how to call the diffrent form in the onclick event