What comes next in the sequence? This is a challenge problem that I am given in class, but I never figured it out. It's just for fun.

What comes next (after the third picture)? At first, it seems like the blue figure is just shifiting to the right. But the third picture messes everything up. 

I've tried just about everything you can imagine (even writing the blue dots as "1"'s and looking at the binary representation of the rows and columns), and I can't get it.

These are the choices.

I would guess (a) or (b), but I don't have solid reasoning.
This is the correct answer. The key here is to realize that the pattern is moving right one by one. So your first column becomes your second and your second becomes your third. 

When they reach the third column, both the yellow and blue dots shift one down and if they are already at the bottom, they go to the top. (They appear at the first column again with these transformations).
This is the correct answer. The key here is to realize that the pattern is moving right one by one. So your first column becomes your second and your second becomes your third. 

When they reach the third column, both the yellow and blue dots shift one down and if they are already at the bottom, they go to the top. (They appear at the first column again with these transformations).