dynamic drop down - pls do help

You could have a look at acts_as_tree to build up a hierarchy of options using a single model
http://wiki.rubyonrails.org/rails/pages/ActsAsTree

So you could have first options: A and B

and as children of A have: C and D
and as children of B have: E and F

You can scale this to as many levels as you like.

Cheers,
James