Conditional Authentication / Authorization Approach

I have the following fairly simple scenario. I have a set of pages that
require users to be authenticated ONLY when certain other conditions
are true. For example...

These two pages require users to ALWAYS be authenticated:
- PageA.apsx
- PageB.aspx

While these two pages only require users to be authenticated on
- PageC.aspx
- PageD.aspx

This is a silly example, but it shows that the condition is not user
related, but rather environment or configuration related.

I am trying to determine a simple way to handle this without having to
write a bunch of conditional logic in every page. My first thought was
to use page inheritance somehow, but this seems like a job for roles,
membership, and authorization using the web.config file to specify
authorization rules.

I just can't figure out how to appoach this example. Any help would be
greatly appreciated!