Identifying variables in experiments

The one variable in an experiment that is purposely changed by the scientist is the________________.

Research Question: Does the type of gasoline put in a car affect how fast the car can drive. I try regular, plus, and premium to see which one makes my car faster.

What is the independent variable?

Does changing the scent of my dog's food affect how much the dog eats?

I try original, chicken, beef, and shrimp to see which the dog eats more of.

Which variable is the part in red?

Does changing the scent of dog food affect how much the dog eats?

I try original, chicken, beef, and shrimp to see which the dog eats more of.

Which of these would be the control group?

Does changing the scent of a brand of dog food affect how much the dog eats?

I try original, chicken, beef, and shrimp to see which the dog eats more of.

Which of these would be a constant for this experiment?

Does the flavor of ice cream affect how fast the ice cream melts?

I scoop equal amounts of vanilla, chocolate, and Macinac Island Fudge flavors into three separate bowls.

Which variable is the part in red?

Does the flavor of ice cream affect how fast the ice cream melts?

I scoop equal amounts of vanilla, chocolate, and Macinac Island Fudge flavors into three separate bowls.

Which of these would be a constant for this experiment?