How do I do this equation in access?

[ivwshane]

Ok I'm creating a table that has two yes/no fields and I need the equation to put in the "validation rule" box that will only allow a user to select yes on either or on one of the yes/no fields but not be able to select yes on both.

So it looks like this:

field a)yes / no
field b)yes (only if field "a" is no) / no

Or how can I make it so a user can only choose the options that I give them (meaning that they can't type anything except the choices I give them)?