In an experiment to test how light affects plant growth, which variable should be kept constant to ensure valid results?

• A. The light level
• B. The type of plant
• C. The growth rate
• D. The weather

Answer: (B)

Controlling variables is essential to show that light alone affects growth. To compare how different light levels influence plant growth, you must keep everything else the same. Using the same type of plant for every treatment removes genetic differences as a cause of growth variation, so any observed changes can be attributed to light. If you used different species, some would naturally grow faster or taller, making it hard to tell whether growth differences come from light or from the plant’s biology. The light level is the variable you intentionally change to test its effect, so it should not be kept constant. Growth rate is the outcome you measure, not something you hold fixed. Weather and other environmental conditions should be kept as consistent as possible, but the key constant in this setup is using the same type of plant.