A student wants to answer the question "Does adding calcium powder to soil make pea plants grow faster?" She adds calcium powder to the soil of 12 plants and does not add it to the soil of a second group of 12 plants. She measures the growth of all of the plants in each group for a month. Which statement best summarizes the factors that need to be controlled in this experiment?
A) All plants should be the same age and health and be given equal amounts of water and sunlight.
B) All plants should be placed in separate, equal-sized pots on the same table, where they receive the same amount of sunlight.
C) All plants should be watered in the morning at the same time each day, and given the same amount of soil and sunlight.
D) All plants should be the same health and be given equal amounts of water, calcium, and sunlight.