Answer:
Part A: The independent variable in this hypothesis is the temperature of the environment. To test this hypothesis, the temperature should be changed to different levels, both warmer and colder than 25°C (77°F), and the effect on the carbonation of the beverage should be observed and compared.
Part B: One variable that should be held constant to accurately test the hypothesis is the initial amount of carbonation in the beverage. If the initial amount of carbonation in the beverage is not the same for each test, then the results may be skewed. Therefore, it is important to use the same brand, flavor, and batch of the carbonated beverage for each test. Additionally, the amount of time that the beverage is left open and the size of the container should also be held constant.