To Do an Experimental Study of the Effect of Smoking

To do an experimental study of the effect of smoking on physical health, we would have to ________.

A) find a group of smokers and compare their physical health to a group of nonsmokers
B) randomly assign people to a smoking condition or a control condition
C) compare the physical health of a heavy smoker to the physical health of a person who has never smoked
D) teach one group of people to adopt good health habits and see if they are more likely to begin smoking than a group of people who have not been taught good health habits