I am sorry if this is such a newbie question, but I can't seem to find the answer... Let's say I have a survey with the following groups:

-Landing block ( L )

-Question Group A ( A )

-Question Group B ( B )

-Demographics ( D )

I'd like responders to be randomly assigned so that each of them has 50% of seeing Question Group A but not B, and 50% of seeing Question Group B but not A. That is, around half of the responders should go through L -> A -> D; and the rest would go through L -> B -> D. How do I do this? Thanks!