[Javascript] Using option box value to set the items of select object

[M. Amin]

Dear All,
 I've a situation in which i let the user to try to choose a country region
like Western or Eastern
and from his decision i list the appropriate country list in the same page,
i'd like to implement it using
Jscript but i don't know how can i affect the <Option> elements of Select
Object using the choosed region.

Any help will be appreciated.

Regards,
 M. Amin