20 POINTS!
The CDC (Centers for Disease Control) is a federal agency that monitors the spreading of various diseases and health issues. They have done extensive research on the effects of tobacco on a person's health.
The CDC does a study on smoking with the hypothesis, "If a person smokes more cigarrettes, then they are more likely to develop heart disease". Which of the following should be a controlled variable for this experiment? Choose all that apply.
1. Amount of smoking a person does
2. weight class of the person
3. state where the person lives
4. Whether or not the person develops heart disease