form_radio() and form_dropdown() .. array data . how to ?

[eluser]Nano HE[/eluser]
Revised.

My CI race registration form.
http://www.ironmanchina.net/active/index.php/form/

When user selected the gender, such as "Male".
How to only show the Male related the race groups at the below "Race Category Select" form_dropdown() list.

Anybody could give me a hand?
Any help will be appreciated.

:-)