A pair of coloured circles moves along the row one place at a time. When a circle goes past the right end, it comes back in at the left. Which row comes next?

The orange and blue pair moves one place right each time. Next the orange circle moves into the last place and the blue circle, pushed past the right end, comes back in at the far left: (a). (b) swaps the colours, (c) keeps both at the right end with the order reversed and (d) starts the row again.