Each wheel is cut into six equal parts and some parts are shaded. Look at the three wheels of each row together. Which wheel completes the bottom row?

In every row the three wheels between them shade each of the six parts exactly once: no part is shaded twice and none is left out. In the bottom row the first two wheels shade the three parts on the left and the lower-right part, so the missing wheel must shade exactly the two parts still left, the upper-right part and the bottom part: (d). (a) shades a part already used, (b) leaves the upper-right part out and (c) repeats a part.