Researchers Wish to Conduct an Experiment Investigating the Effects of Smoking

Researchers wish to conduct an experiment investigating the effects of smoking on math performance. They place an ad in the national paper and invite smokers of all ages to participate. Such a process allows

A) each smoker to have an equal chance to participate in the study.
B) the researchers to infer cause and effect.
C) the researchers to generalize the results to the population.
D) all of the above.