Final answer:
The amount of sunlight, room temperature, and water were held constant to isolate the effect of sunlight on plant growth.
Step-by-step explanation:
In order to conduct a valid experiment, Gary held the amount of sunlight, the room temperature, and the amount of water constant for both groups.
By keeping the amount of sunlight constant, Gary ensured that the only factor influencing plant growth was the difference in sunshine exposure between the two groups. This allowed him to accurately measure the impact of sunlight on plant size.
Similarly, by maintaining a consistent room temperature, Gary eliminated temperature as a variable that could affect plant growth. This ensured that any differences in plant size between the two groups could be attributed to the varying levels of sunlight.
Finally, by providing the same amount of water to both groups of plants, Gary controlled for the impact of water on plant growth. This allowed him to isolate the effect of sunlight on plant size without any confounding factors.