In the first experiment, which variable is manipulated by the researchers?

• A. Time since first exposure to cane toads.
• B. The amount of rainfall.
• C. The color pattern of snakes.
• D. The density of predators.

Answer: (A)

In experiments, the researchers deliberately change one variable to see its effect on the outcome. Here, they set up different groups with varying amounts of time since first exposure to cane toads, so they can observe how responses change as more time passes. That timing is the manipulated variable—the factor the researchers control directly. The other options describe conditions that aren’t being actively varied in this setup (rainfall), inherent traits of the snakes (color pattern), or a different experimental design (predator density), so they don’t represent what the researchers are adjusting in the first experiment.