In each wheel a block of neighbouring parts is shaded. Watch both the size of the block and where it starts. Which wheel comes next?

At every step the block gets one part bigger (1, 2, 3, 4 parts) and its first part moves one place clockwise: it starts just right of the top, then one place further round, and so on. The next block has 5 parts, starting one place after the last start, so it wraps round past the top: (a). (b) grows but does not move on, (c) moves on but does not grow and (d) moves on two places.