The question is in the details?

Question

A biology student was having difficulty growing houseplants at home. The student thought that the problem might be due to the plant food that was used. Three identical plants were purchased, and set side-by-side in a window. Once a week, each plant received 50 mL of water. The water for Plant #1 contained 5 drops of Brand X plant food. The water for Plant #2 contained 5 drops of Brand Y plant food. The water for Plant #3 contained no plant food. Which plant serves as the control for the experiment?

Answer 1

The control is plant #3 as it contains no plant food.  Plants 1 and 2 are each being tested with a different brand of plant food, obviously to see which one was more effective.  Therefore, by having plant 3 with just water, and no food, this is was the control to ensure there were no other variables influencing the results.