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