Melanie wants to observe the effect of two new fertilizers on the growth of plants. Three identical plants having a height of 15 cm were used for the experiment. No fertilizer was added to the first plant, 100 g of Fertilizer A was added to the second plant, and 100 g of Fertilizer B was added to the third plant. All other growing conditions of the plants were kept constant. Which step of the scientific method did Melanie do here?