A girl has an idea that green plants need sand in the soil for healthy growth. In order to test her idea, she sets up two potted plants. She sets up POT A with sand, soil, and water on a windowsill in direct light. She sets up POT B with just soil and water on a windowsill in direct light. Which plant is the control?

POT B

Explanation:

In an experiment, the control group is the group that does not receive the experimental treatment. In this case, the experimental treatment (independent variable), which is the variable that the experimenter changes is the SAND that the girl believes is required for healthy plant growth.

Since the experimental treatment is the SAND, the control group will be the group that doesn't receive the SAND, which is POT B in this case, which received only soil and water. Note that, SAND is the independent variable in this experiment.