Final answer:
Sunlight is the independent variable impacting the dependent variable, which is plant height. Control variables like soil type and water amount are kept constant, while the experimental groups receive varying sunlight levels to test the hypothesis that more sunshine leads to larger plants.
Step-by-step explanation:
The amount of sunlight a plant receives can indeed impact its height. In an experimental setup to test this, sunlight would be the independent variable because it is the factor that is being manipulated. The height of the plant, which is the outcome being observed, is the dependent variable.
The two constant variables could include the amount of water and the type of soil used, as these must remain the same to ensure that any differences in plant growth are due to sunlight and not other factors. A control group design would involve a set of plants that receive a standard amount of light that is not manipulated. Experimental Group 1 could be designed to receive more sunlight, while Experimental Group 2 could be set up to receive less sunlight. This would help to illustrate how different levels of sunlight affect plant growth.
In regards to Gary's observation, the most reasonable statement of his hypothesis would be (b) 'Plants that get more sunshine grow larger than plants that get less sunshine.' Gary insists on using the same container size to ensure the growth differences are due to sunlight, not container size. Similarly, he ensures all plants receive the same amount of water to control for water as a variable.