A test is set up to test which brand of soda creates the biggest foam explosion when adding a Mentos candy.  Three different sodas were tested. They measured the height of the foam eruption.   What is a controlled variable (constant) for this experiment?

If I study for my scientific method test, then I will have a better test score. Who would be in the control group?

Research Question: Does using a higher quality of gasoline in a car effect how fast the car can drive. What is the experimental group in this experiment?