In Each Case, Which of Mill's Methods Is Being Used

In each case, which of Mill's Methods is being used?
-A cook tries adding five different ingredients to different batches of cookies in an attempt at discovering a new taste. Ingredients are added one at a time per batch. All the added ingredients that contain chemical X produce cookies that are as hard as rocks. The other chemicals do not produce this result. The cook concludes that the chemical X makes the cookies hard.